I'm sure there are some "data harvesting" reasons, but honestly, the simplest is likely the truest:
Most people aren't computer-savvy, and having an app is much easier for most users than going to a website (either directly or through a bookmark that they probably won't ever be able to find again).
One must remember, always and forever: most people aren't us/you. Just because something is easy for you to do doesn't mean it's easy for everyone else.
Is it dumb for me that T-Mobile has an app that just goes to a webview that I could get through my phone browser? Yes. Is it dumb for my parents? Absolutely ten thousand percent no.
The value (in terms of money made/saved/protected) that a company gets from having an app instead of a website only is probably ranked in this order:
1 - ease of use for the majority of customers, reducing tech and customer support calls, angry customers, lost goodwill, bad reputation
2-99 - same as #1
100 - data harvesting