The main reason IR exists is to help you manage your roster when players get hurt. Imagine having a star player who's out for several weeks - without IR, you'd have to either cut them or keep them on your active roster, taking up a valuable spot. But with IR, you can stash them away and free up space for another player who's actually playing.
It's kind of like when you're playing a game of Mario Kart and you get knocked off the track - you can restart from a certain point, rather than having to start all over again. IR gives you a chance to restart your player's contribution to your team when they're healthy again, without having to sacrifice a roster spot.
