It's about friction. you could give users a PO box address and ask users to get out an envelope and write a check, and say if users really wanted to pay they'd do that. But we all know you're gonna lose out on a ton of money if you make it that hard for people to give you money. So it's about how much friction there is, and for some, an app is preferable to some website where I have to wade through a bunch of crap. Depending on what it is, if it bounces me to Safari, I might just get distracted and "just do it later" aka never.