Sometimes ...ok most of the time I'm bored. and having been a creative person my whole life I have the urge to create something. If I want something funny I usually add my face. I don't take my photo in thought of using it for a specific ad or project, I usually have something already. Because those other times I want to create something and want to take photos - I take them of myself. I think at some point in everyones life we've all pointed the camera at ourselves, photographers have been doing self portraits since the beginning of photography.
At digress... Photoshop is fun - and if you look at any celebraty photo or magazine, photoshop was most likely used to edit the photo to certain standards, but that's for another post and soap box.
Here Photoshop is fun.
Here is a weird tutorial - adding a face to a different body.