i have seen a few vids that include retopology section but i am wondering that, in what circumstances we need to retopology our mesh?
When you want animate your sculpture in other programs you can
put the faces when needed for good rigs
When you want more details in some specific areas and less in others
without reach zillions of polygons
When you want nice UVS
Retopo is everything! 
ohhh i see thank you very much
by the way, i am new to this forum!! also new to zbrush!
welcome to the magic kingdom