Things like buying the console, buying add-on content for games, etc. People have a membership, and want to make more use of it- though MS likely doesn’t mind people keeping it to the basics, since they don’t make huge bank off of singleplayer gamers anyway. I won’t even deny, I’ve bought small microtransaction items in a few games I only access through Game Pass - but I’m sure other people do far more often than me.