I strictly use git add -p.

for one, it lets me create small patches of related stuff. There's nothing wrong with major patchsets in general but it makes it harder to cherry-pick little fixes to old stable branches for example

two, I notice other developers making me do the work for them because crap sneaks into their commits, like debugging statements or accidentally removed hunks. Instead I have to do "git add -p" when reviewing their commit.

Essentially, it's a first pass staging area you can review what you did. beautiful.