It does have a killer app: The form factor.

Lots of devices do lots of things that the iPad does. But they all have screens that are too small, or keyboards, or other things that make them too thick.

ipadOS apps aren't as powerful as computer programs, but they're good enough. And when combined with the iPad's form factor and simplicity, there's a reason that — according to Wikipedia — Apple's sold a half-billion of them.