Because the bag comes free with purchase and it can carry things around. I don't think there is any "Oh look, I shop at Victoria's Secret, I must be wearing super cute panties and bras" about it. If we're into a guy, when the time is right, he'll know way more about those lovely things than just seeing a bag could tell him.
I hate that ugly bag! It just says "Victoria's Secret" on it and its pink with stripes, its a hideous looking bag. But girls really wear that to let everyone know they shop in VS and theyre "hot". I shop there but I don't strip naked and show off my bra >.<
for some girls it could be partly that, yes. otherwise the girls just really like the bag and want to re-use it if its durable. some girls are really into brand name stuff too, so they may like carrying/wearing things that have a label on them