Ease of use, easy of discovery, learning curves, etc.
General -> VPN -> add configuration and entering
- Server
- remote ID
- Username
- Password
This is less setup than adding an email account.
They may have heard of so-and-so app, install it, and boom, it works. That's very appealing, especially if it's a custom protocol that is supposedly harder to detect.
Or maybe their grandkids have set it up for them, and don't want it to be any harder than it needs to be. Yada yada.
It's not really hard to come up with answers like this. The two options aren't "complete expert" and "blithering idiot".
I regularly jump between UK, US and France in my VPN app.
The only sensible reasons I could see for an app are either to deliver the content to the user in the manner the user is most accustomed to, or to be able to integrate billing as in-app subscriptions, etc.