I'll admit to some vanity about how I look working out. I want to look fucking fierce - and not fatty "fierce" but "holy shit that chick looks like she'll kick my ass if I interrupt her set" fierce. I work out to become badass; I want to look badass while I'm doing it.  This is less dependant on buying expensive clothes than it is just not looking like a slob, though, which means I've already got a leg up on fatties because they always look like slobs.