A little extra time spent writing a good commit message can save a lot of time when trying to understand the commit later.
If the commit message and the diff disagree then both are probably wrong
A good commit message helps both code review and understanding the commit later.
When getting a bug report that something broke on an upgrade between version X and version Z, good commit messages - and subject lines in particular - can make all the difference when scanning the revision history between those versions to find possible candidates that caused the breakage.
In cases where one stumbles upon some old code that looks suspect it can be hard to know if there is a reason. The message in the commit that introduced that piece of code would be useful if it provided enough details to help assess if the reasons the code look they way it does still are valid or if surrounding environment has changed since it was written.
If there is a part of the patch that doesn't correspond to what is described in the commit message, that part tends to be the one that is most likely to contain bugs.