[matthewwarren]

> but in these cases it seems their purpose besides explanation is to imply that there is a good reason the code is the way it is, and that you had better have a very good reason to change the code (and the accompanying comment). In other words, they serve to also guard against change, in parts of the code which should not change often.

Interesting, I'd not thought about that 'side-effect', but you're right, it does guard against that