It still has more upsides than your average X as a service. Beyond what the OP already said, it lets you jump right into a game without having to wait for it to download, which is pretty big when games regularly take >100GB of space, even more so if you like to switch between different games often.

If you try a game and decide you don't like it, you may as well end up using up less bandwidth than if you had downloaded, not to mention zero waiting time (unless there's a queue, but usually there isn't assuming that you have an actual paid subscription and it isn't an absurd peak demand).