Boring answer time...

They can be a pain to do, but self-evaluations are useful to me for several reasons. I want to make sure employees are not only primarily focusing on their assigned tasks, but I also want to make sure they aren't doing more than what's appropriate or going beyond their training or expertise. Many employees will not let you know when tasks have been delegated to them from someone else, particularly a superior. This can be for many reasons, but very often they simply think it's "just part of their job". Self-evaluations have led me to discover that people were doing other people's work, and it helped me reassign responsibilities to their appropriate employees.

I also view evals as a sort of tune-up, helping people properly prioritize their tasks. If I read an evaluation and someone spends most of the time talking about a secondary responsibility, I can then sit with them and make sure they understand their primary responsibilities. I don't just wait until annual evaluations to do this, but unfortunately many people will not tell you these things in everyday conversation. They sometimes need the opportunity to put it down in writing to really clue me in.

I will admit that most of the time evaluations don't bring anything worthwhile, and it is just a chance for people to tactfully brag; but sometimes it's the only way to find out certain information.