[Espressosaurus]

When you find a bug later on related to a specific commit, the "why" in the commit can help explain the "what" that resulted in a bug.

It's all in addition to the changelog and other documentation if following a defined release methodology.

Personally I like having the "why" in a comment (with a pointer to the bug tracker item) if adding code. That doesn't work as well when deleting code, and for that, the commit message works as well as anything else.