Short answer: IMPROVES IT.
(Sorry for shouting.)
The camera shows us different things than the mirror. I have learned a great deal about proportion and outfit cohesion from studying my own (and others') photos -- things I could not learn through descriptions alone. Each body is a bit different, so simple "body type" categories don't always tell the full story. The information you get from photos helps you understand your own body type modifiers better, and see what actually looks good on you, as opposed to what you might imagine looks good.
Also, the knowledge that you are going to take a photo can encourage you to step things up a notch. Even when I don't post photos to the group (I sometimes take them without posting) I might make more effort if I am taking a photo than if I am not. Especially for my work-at-home life where the temptation is to give in to wearing PJs or gear for most of the day.
I didn't even have a style before I joined YLF. Posting pictures was a crucial part of figuring it out and beginning to refine it.