Presumably because it's a relationship. And you do stuff for each other. Sometimes stuff you don't necessarily want to do.
Blowjobs don't just drop outta the fucking sky.
I'd prefer not to carry the purse just because I don't like holding onto extraneous stuff (why bring it if she doesn't want to carry it?) but this man knows what's up.
Also, sometimes if I have something I need to put away or am wearing pants with no pockets, she'll let me put my wallet/cellphone or whatever inside the purse she's carrying. Relationship teamwork right there, so yeah I'd hold the purse.