I love statements like the one of the OP. "All women do some sort of stupid stuff" - just because he knows one or two that are that way.
I, on the other hand, don't know any woman who buys expensive, branded bags. That's mostly because my social circle does not really include the fashion addicts, it includes mostly the backpack + sweater & jeans type.
So, I don't know why women supposedly only buy branded, expensive bags. The reasons may be similar to why some men only buy expensive cars though: Impressing other people with this expensive item; showing off; and getting some self-esteem because they've got something with supposedly high quality.
@Alaris: I don't think that anorexia and shopping mania have the same psychological reasons.