People (and I'm one of them) usually say why instead of what because in general the what can be understood if you can read code. But obviously no hard rules, if I write something twisted because I had no idea how to do it better or had the time, then I'll write what it does. It's not perfect, but nothing is.

Same goes for comments vs commit messages. It's a fact comments get outdated and then you have an even bigger problem whereas a commit message is always correct at the time it was made. But obviously again, no hard rules. Sometimes I feel it's better to write a comment, e.g. if it's something that is really important and won't change a lot.