anything that is not in the commit message or in the commit itself is practically useless. documentation covers how to use a thing. it usually doesn't cover why a change was made. that why also normally does not end up in code comments, and even less so would be aparent from the change it self. the commit message remains the only place where the why can be documented meaningfully and along with the change where it is needed.

at least in my current project working with legacy code that i am not familiar with, the why is the most important detail about a commit.