I’m guessing there is a benefit to buying the app, but the app description doesn’t really specify.
It’s the iOS app btw.
It’s basically the dedicated mobile interface and the math keyboard. The mobile interface is generally a bit faster because it only has to request plaintext results from the server, while the site re-renders the page every time you submit a new query.
Better step-by-step support and a problem generator are two things that come to mind.