The PR should be the smallest unit of integration (this works and builds and is ready to merge to the next steps), but the commit is the smallest unit of any progress at all. Ideas don't come fully formed and ready to compile. Progress sometimes includes back tracks and experiments. Good commits say things "hey, I learned from this thing that wasn't working and that's what pushed me into this next direction". They document the journey, why a specific path was taken, what obstacles were in the way, what other paths were explored and dismissed.

Some PR authors can capture a lot of that in a PR description as well, but commits tie that to specific context of the code at a point in time in ways that a PR description often can't, without linking to commits (in which case the commits again speak for themselves).

But yes, not everyone can or will write good commits. I see that partly as a tooling failure because our tools themselves like GitHub PRs don't encourage it, often fail to reward it. I've seen plenty PRs full of commits named only "WIP" and "Fix" and "oops", but the best PRs tell a story in the commits, have meaningful descriptions on each commit. I would love for our tools to encourage more of those because I think they are better PRs; often easier to review PRs and enough good PRs like that form a string of documentation that you can search through (through git blame and git log if nothing else, but that's still a lot of useful research data). (If you keep that data, I know a lot of people like squash merges because they distrust the git DAG and how many tools show ugly or hard to read "subway diagrams" for it instead of simpler collapsible views. But that's another long conversation.)