There are many reasons for painting self portraits, not least being the continuation of a long tradition of self-portraiture among artists (just think of those by Rembrandt and Van Gogh). Then there's the advantage that it's the one model who's always available, at any time of the day).
I've been hooked on self portraits ever since I first attempted one (which was not a success, though my second self portrait I framed and still have on display). I don't paint self portraits for any narcissistic reason, but for the challenge. After all, if I can't capture my own likeness and a feeling of my character, how can I attempt to get someone else's?
I've done self portraits in charcoal, pastel pencils, watercolor, and acrylics. The results have varied from as realistic (in terms of color and likeness) to strongly Expressionistic. From pleasing (the self portraits I show others) to strange (the self portraits few people see). I regard getting a feeling of character more important than a photorealistic likeness, for which I personally prefer using a camera.
I rarely set out with something specific in mind, other than to paint a self portrait, and just let the painting evolve on the canvas, following the mood I'm in. I use a mirror set behind my easel so I could see my whole face and shoulders, plus a small mirror attached to my canvas board with a bulldog clip. The former is to get the overall shape, proportions, tones, and shadows. The latter for seeing detail in specific features.