Why are there so many apps that could be websites?

Seriously. I don't want to install something on my phone when the dev is just using a WebView, if that's what it's called. When the app is basically just a website with the browser hidden.

What's the reason for that? To attach the customer? To sell the app for money? Is there more ad revenue that way? Do you reach more people?

(Are there any good reasons for it, too? Security, maybe?)