Far too often the commit message says "what" while the branch name says "why."

Just recently I had to triage a recently discovered bug whose symptoms disappeared about 4 years ago to determine if the bug was fixed or just masked. This was code that I was completely unfamiliar with because the other engineers that might have looked at it were otherwise indisposed working on things that were known for sure to be issues.

The "what" told me the specific change that masked the bug and the "why" informed me of the feature that this supported. That allowed me to generate a new test-case that showed up in the current version. Without both pieces of information I would likely have had to spend a couple of hours inspecting code changes to do the same.

If you can't trust your developers to write acceptable commit messages, why are you trusting them to write code?

Because you're less in the nitty-gritty when naming a branch, so you're more likely to name it with something referencing the large feature you're working on.