People will pay for these cards on the Marketplace, even though they drop for free during games and don’t materially exist, strictly speaking. The most obvious reason to collect Steam trading cards, though, is to sell them. Whatever brings the room together, right? Maybe they build virtual houses with them, or attend Steam card conventions to show off Achievement badges, or Card cosplay as their favorite foil drops. The 'why' part is as immaterial as the cards themselves. Believe it or not, some people collect Steam’s virtual trading cards.