There's a great clip of Peter Hurley describing the importance of self-portraits, found here, but the evaluative upshot is they help the photographer recognize facial "flaws" and how to photograph around them. Self portraits also serve to remind the photographer of how daunting it can be to be on the other side of the camera something which can be easily forgotten. It's also a stress free way to try out different lighting setups and play around with gear. After all, no one else's time is at stake so you can shoot the subject as long as you'd like with relatively little complaint. 

It's also a good way to fill up some down time, especially on days when leaving the house sounds infinitely terrible. There are some unique headaches involved, getting correct focus is always fun when you're not at the camera, but ultimately the time spent doing a self portrait helps inform your photography when shooting other subjects.