Every photographer understands the impetus behind the self-portrait: not some formal tripod and time-delay creation, but the quasi-accidental inclusion of oneself in a composition, in the form of a shadow, perhaps, or a reflection.
Photography is a curious art-form, for many reasons. Unlike most forms of artistic representation, it requires relatively little ability to translate sight into the moment of a hand. Instead, it required just one firm touch upon a button, under conditions that are recognized as being correct. The difficulty of affirming the originality of that act may partially explain the motivation behind the photographer’s self-portrait.
Mostly, however, I think of it as a playful interchange between photographer and observer. Whereas the two are normally in a conspiracy with regards to the subject (at the same time as the photographer and the subject are in conspiracy against the observer), in this case, there is a kind of simultaneous mutual revealing, which is very artistically and emotively interesting.