I'd suggest that for a lot of users, Procreate is the closest thing to a "killer app" that's iPad only and emerged organically from a third party.
Using the Apple Pencil with either app gives me the benefits of analogue work and the benefits of digital work. Handwriting helps me remember things, but since the product is digital, it's trivial to move/undo/etc.
And Procreate + Apple Pencil is way better than any other art solution I've found for digital art. Granted I've never tried a drawing tablet with a screen, but an extremely cheap option is still $200. For one that works as well as the AP + iPad you'd probably be looking at near the same price point.
I'm an hobby artist and have played with various drawing programs on the ipad (and gone through a few mid-level wacom tablets before that) and Procreate is simply amazing. It's an app that couldn't exist without the ipad and it massively increases its usefulness. Procreate is good enough that I don't feel the need to bring any further art supplies (even a sketchbook and pencils) when traveling.
Another thing that IMO is way better on iPad/Pencil is PDF editing and annotation. I happily pay for a subscription to PDF Expert[0] and I just use it for school.
And given that it was a non-pricy one-time purchase, and not a subscription, why would they regret it? The cost per hour of usage is only going to keep decreasing over time for them, as long as they keep using it in some capacity occasionally.
I don’t regret it because I can see that what I paid for is worth the value of the app despite not using it as much.