Here's a complete newbie question....

Why use a messenger bag?
I mean, opposed to conventional backpack, or smaller backpack type bags.
I can see the advantage of not having the bag attached to your bike (panniers, bar bags etc).

Are they REALLY comfortable?
Simply hold more stuff?
Don't move around while riding a bike? (seems like they would)
Simply history of bike messengers?
...the style that goes with it?

Do you use them simply to carry random stuff, or are you really messengers and need to carry larger sized paperwork, documents, and packages?

Just curious... I'm gonna need a bag of some sort for commuting, and was wondering why messenger bags are so popular in the bike crowd.