I think Otis Gospodnetić has it spot on:

IMHO, commit messages are an important piece of the communication puzzle, and I feel they are often neglected or completely not even seen that way. Commit messages (that are distributed/delivered to co-developers via email or some other mechanism) help a developer keep others in the loop. They help co-developers stay up to date mostly passively, without having to proactively look for information about recent changes.

This is especially true with distributed teams but I’ve always found it really useful for co-located teams as well. Being co-located you can provide updates verbally but it’s so much more efficient for people to keep track of the commit messages in their own time. It’s like the interruptions of IM vs the when-you’re-ready of email.