All of this, although 30 commits is getting a bit extreme. If you can write 30 commits, each with its own tests, your PR could almost certainly be smaller while still delivering value (not dead code). But I regularly end up with 10 commits in a PR.

One major benefit is bisecting. I often don't find I've broken some automated test until I push my "finished" code for review, and incremental changes, each of which independently pass all tests, can be easily bisected to discover the problematic change.