What is the point of an employee self evaluation?

It lets the most interested party do the data gathering that the manager would otherwise have to do. It also ensures that the employee is thinking about everything that they're being evaluated on more than just after the annual eval is done.

They're a good thing - it gives you a chance to tell the boss about everything you do, especially the stuff she doesn't notice. Spin it freely, and use quantitative stuff wherever possible.

Because they don't do a good job monitoring you and they want to take advantage of your honesty and have you give them a ceiling to your evaluation.

Or, they want to correlate several data points (objective measures, peer review, management review) to find outliers and improve the reporting process.