why do layers have -1?

[bLawless]

Why do the layers (under the tool menu) have -1?
I use the layers all the time for blend shapes on faces. But I go from neutral at -0- to an expression at 1. When I am in a meeting if I am not careful I take the slider to the left too far and the face looks terrible.

Maya blend blend shapes go from 0 to 1, why doesn’t zbrush go from 0 to 1?

Are there a good number of users that find -1 useful? Would people that use -1 post how they use it?

Thanks

[beta_channel]

I use -1 for eye direction. Both Vertical and Horizontal.

0=staight
1=right
-1=left

I also use a slight negative value for anticipation before changing expressions.