I usually just buy any expensive vehicles or vehicles with unique abilities. I like collecting vehicles, but mainly the higher-end ones. If they added a new 30k vehicle there's a decent chance I'd skip on it but the lack of updates as of late has had me expanding my horizons I guess. I like having things to have them so I think I'd say it's for my collection, although if I couldn't actually try out the things I bought I'd probably be a whole lot less interested.
I like going for good vehicles right now, or just vehicles that I may have fun with. For example the Warhawk or Overdrive. I don't think, that I'd ever start collecting vehicles, because of how much time I would spend into that and, I don't vehicle, that are not fun to use. I mean, I'm really just playing mad city, for fun and not to flex on anyone.