Bugs can be just as important as source code - if a line of code is changed and all the comment says is "bug 1234", then hopefully bug 1234 has context about the scenario that was broken, a history of a conversation between QA and development, and signoff from QA with whatever was verified.
Sure, this might fall into "write a good commit message", but obviously that doesn't always happen, and having the historic knowledge of the bug tracking database at your fingertips is important.