Writing good commit messages can be good self-promotion. You may not realize who is watching the repo and this is an easy way to differentiate yourself from you colleagues and make your work appear interesting and important.

Also, I have found that writing commit messages often forced me to rework a poor solution whose weakness became apparent only when I tried to explain it. In other words, often the value of the commit message is not the message itself but rather the process that produced it. It's a bit like musical scales. Almost no one is performing scales at a concert but also almost no one is performing without scales as a part of their regular practice.