Why I fast switch:
1. Adjust my crosshair for either a new target, or a missed moving one.
2. See what's going on around me, or perhaps make a turn faster. (I have a low zoom_sensitivity_ratio)
3. Move along faster, for example, if we are rushing, I don't want to run slow with awp, I'll switch to a pistol instead, then, if I see somebody far off, I'll switch to awp to shoot them, or if I'm out of pistol bullets I'll do that also.
4. If I non-lethal somebody with awp, and know 1 deagle bullet will take them out, I don't want to have to waste the time with zooming in and possibly dying from it, I'd rather shoot one bullet with my pistol, switch back to awp and find a new target.
Those are the main reasons I do, plus once you've done it for a while, it just feels normal. It's the same thing with buying full ammo for me; now I know I won't use 40 awp shots in a round, but I just feel safer knowing I have them.
InSaNiTy
( Pro Fast-Switch )