It had a bit of narrative (killing the corrupted son of the traitor out of mercy and to save the world, killing the corrupted angel like beings because they are a threat to humanity), but you’re right, what really drives you is the fun gameplay loop and the challenge of escalating difficulty.
When I still had an iPod Touch many years ago, that only worked after Jailbreaking. The way they lock down the file system on iOS has been one of the worst parts of the Apple Walled Garden in my opinion.