I mean, not that much reason. Maybe you want to save storage space, might be cheaper to keep the Ps4 to play ps4 games than to have to buy external storage (can you still do that?) for the ps5. Although since the ps5 plays the ps4 games better that might be more important. Maybe you want a blu ray player (ps4) for your bedroom tv while you keep your main console (ps5) in your living room with your main tv. But like I said, not much reason to keep it.