No matter how many times I read explanations, or watch youtube videos, I don’t see what’s so important about retopology. What does it do? Why do you do it?
small_orange_diamond
Retopologizing a problematic topologised mesh will help in proper animation of the mesh. If your mesh topology is not correct (quad polygons) then when you try to animate it you will notice problems such as holes or weird extrusions in the mesh. Furthermore, you may even notice these type of problems when you render it in an external application.
So it isn’t essential?
Retopology - enables you to re-draw the basic form/mesh with less polygons where you don’t need them, and more where you do, so if you sub divide you dont have to do it 9 times to get enough detail in say, the eye area.
also essential if the mesh is going to be animated.
Makes for a much easier to work with model too - the smooth tool works better as well.
It is essential, but you dont need to do it if you have a powerful PC (so you can step up the sub divisions) and only need to render in Zbrush (for 2D images, etc…)
to explain this as basic as possible, say you have a sphere, and turn it into a face - you would have a lot of poly’s at the poles at the top and bottom - that is bad - you need more poly’s in the mouth and eye area, not the top of the head!
It is quite essential…would you want say create an animation and have your character walk for instance and as he/she is walking produce weird creases at the elbows or any other parts where you haven’t done correct retopo? Think about it, you will see that though it takes some time to retopo a mesh it is a very valuable thing to do…would you like having your character having weird extrusions where none is supossed to be??
+everything Keeper said 
I will assume that you know nothing about animation, or box modeling.
Quads are great… they get sculpted really smooth and you can do really great things with them…
Rectangles are horrible… they subdivide in a wrong way and the brushes get distorted when using them…
Whenever you sculpt from a basic form (a cube or a sphere) some stretching occurs, so your original quads will become rectangles, so you will need 4 million polys to give detail to the nose, neck, arm…
If you retopo it, you can make all those rectangles into squares, and if you more detail in a zone (the eyes, lips, hands…) you can give it in the retopo.
Suddenly your 4 million poly character becomes a 500k poly character… so you can give it a LOT more detail…
Hope this can help to make it clear
Rectangles… 
Triangles are the plage… I forgot to mention them
Allows you to sculpt freely. You can greatly warp and distort your initial mesh, and before getting too detailed, do a retopo. Then your detail will be evenly spread amongst the new shape as you sculpt. Getting the mesh with even quads before doing the hi res details is very important sometimes.
Another thing it will allow you to do, if you want, is to not do even quads. You can strategically place more polys in areas that you want more detail and less in areas that won’t need it. If you’re doing a head bust with no hair, for instance, you could place more, smaller polys around the face and less, larger ones around the top and back of the head where a denser mesh isn’t needed to provide adequate detail.
So in these ways, it allows freedom and efficiency.
You can also use the retopo tools to create new subtool meshes. ZSpheres or mesh extract are not always the way to go when just working in house. For instance, adding a cape, or something that either isn’t round in nature or doesn’t “stick” to a character is easy through retopology.
I’ve seen many videos where retopo is done in ZBrush on greatly subdivided meshes with the topology tools of ZBrush.
So you can surely do after a retopo as well, no??
To me the biggest benefit si when you have been sculpting and find that you need to add more detail in certain areas, but don’t have the underlying geometry to handle it (around the eyes on a face for example). You can retopologise giving yourself more geometry in that area only.
The geometry flow is important if you are planning to move your mesh at all. That inlcudes animation sure, but also includes posing.
Seriously, learn it, try it, get used to it and you’ll find that it is an invaluable tool in ZB.
Yes many (maybe most) people do retopo as a last step over a high res mesh. When the polygon distortion doesn’t get so bad that you can’t do your details, you can finish the sculpt, and then retopo just to even out the polygon distribution of the detail. Which creates a cleaner look. Going in after a retopo and doing some extra detail can usually be a good thing though.
Again, it depends on your final goal. I use ZBrush sculpts for art, and often do not need to retopo. In fact, I just recently learned how to do it a couple months ago, and I’ve used ZBrush for more than a year and half. I had some trouble figuring out how to get a clean projection, but I think I got it down ok now.
It has since come in handy on a couple sculpts where I distorted the initial shape of the mesh, and it truly improved the final result. 
Hmmm, what if you did a retopo in like say the mid sculpting process, especially if your mesh had a problem with a few triangle formations. Then the brushes would work better on the correct fully quad mesh…and you would continue sculpting smoothly your final mesh, no?
By the way rooky i hope that by now we have highlighted the importance of retopology for you and answered your question 
Yeah, retopo is a great way to correct your mesh, both in topology (getting rid of tris, improving edgeloops, etc) and in evening out your polys. The more even the quads, the more even the detail.
The three signs that it’s time to retopo are…
a) you’re having trouble getting detail in some areas where needed while other areas are just fine
b) your mesh topology is not suitable for ZBrushing (has n-gons, too many tris or poles, etc.)
or…
c) you’re done with the sculpt! (and want to retopo for cleaning it up or preparing for animation)
Also, I wouldn’t hesitate to study retopo tools in any software you use after you get the hang of doing it within ZBrush. It’s a good thing to know in 3d as a whole. I also use 3d Coat and modo for instance, which have very useful retopology tools.
My guess is ZBrush will have some really killer tools very soon. It’s been requested a lot, and is important for the sculpting workflow. Plus they’re freakin’ geniuses. As is, it works just fine, but takes more clicks laying it out point for point.
In ZBrush you can avoid having to retopo if you use ZSpheres with the adaptive mesh option. But it does mean having to shift the surface around with the Move brush in order to make sure that you have enough polygons in the areas where detail will be added.
It just takes a little planning.
Yeah i checked out 3d Coat and it looks like a really promising software for retopo.
Thanks for all the info anyways, appreciate it
As you can see with retop I took the first Model, which contained 6 subtools, and merged them into 1. (body, feet, head, eyelids)
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That means I can sculpt across the Subtools as it’s now 1 single tool - So do a Retop if you have many subtools and need it to be one single mesh!
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I also don’t have to go so high up in the subdivisions as the new topology follows the curvature / silhouette of the model much better.
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when on the lowest level, the model also looks much better (for game engines)
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this model will now also handle much better in animation.
- The first model’s topology isn’t THAT BAD, but look at the knees for example (first pic), and then look at 4th pic (below 1st in RED)
hope that makes it better to understand.
Attachments
So … is making the retop something that zbrush can do automatically, or is it a manual job?
It is manual and can take quite some time depending on the model and your skills, but you can keep existing topology in some parts of the mesh if they are ok.
I was being taught to retop in Max using the graphite modelling tools, just wondering if that is the ideal way of doing it or can it be done inside zBrush?
I myself wondered what the point of retoping was… I can see why now now thanks to this thread.
Cheers