There is also the effect placebo. If you like the way your cars feel and look, you will play better. So that is one incentive to get items you like for your inventory. Another is to collect as many items as you can, sorta like Pokémon. Trading just adds more layers to an already fun game, casual though they might be. For designers and traders and the average player, trading is a very fun and important aspect of RL. Sweaty players might not know or care about it, and then mock those who do.