Bonus: A game you no longer play could still net something on the second-hand market, or maybe you’d trade it with someone. I know there was a group of people at my school that collectively had like two or three copies of the various Pokemon games they’d pass around, exchanging and loaning them on the fly.

Steam Family Sharing is a thing, but not quite so trivial to set up as handing them the cartridge. Never mind about reselling digital copies of games.