I have to shift my brain into totally different modes between programming and reviewing/testing/version-controlling, and git add -p is an important tool. Maybe you're disciplined enough to keep a queue of everything you'd like to do in your head at once, and only stick to one task at a time, and shift into review/test/commit mode between quantized chunks. Me, I just go in, hack for a bit, and when I run out of ideas, I shift modes, read the git diff, break out my work into chunks, and then add -p/stash/test/review/commit chunk by chunk.

If you want all of my commits to be functionally and semantically separate and individually tested, I can give that to you. git-add -p is just an interface between that well-disciplined software-engineering expectation and what my brain actually does when it gets into flow.