It can be a shock when you actually see what they look like. Usually it's a way different picture than what you had in mind (like, if you imagined them as someone who looked like they had a life away from the Internet as well as one on it, they usually end up looking like your average person).
Also, different names can give you different images. Take the name Theodora, for example. You might not quickly come up with the wife of Byzantine emperor Justinian, but it would still be way different than what a person named Theodora actually looked like.
Anyway, I'm just saying that writing reveals a whole 'nother side of you that your real-life friends probably would never discover.