Why messanger bags?

[7rider]

this may be a silly question but why do people use messanger bags like the timbuk2s, crumplers, and chrome bags? I have a backpack and panniers for groceries, yet I am interested in a timbuk2 bag from rei. I don't even like velcro, or the excessive weight it puts on one side of my shoulder. Someone help!

Ah, hah! REI's advertising and mind control is working!

You use a pack and panniers. Those work fine for you.
You don't like what a messenger bag offers (velcro and asymmetry), yet, you are strangey compelled to go and try one out.
Hmmmm... (rubbing chin thoughtfully)....

Yes! Someone please help the poor gal!!