When personal photography was first becoming popular, it was mostly used for experimentation and artistic expression, like portraiture. Over time, as costs decreased and fidelity increased, photos gained a second function: they became a system for people to store their memories. And only very recently have we begun to experience the third major function of photography, and I think it’s far more important than the other two: photos for individual communication.
http://dcurt.is/photos-for-communication/