Choosing to do an app is quite often less about the capabilities (of an app on the phone, versus a website in a mobile browser) and more about discoverability and market reach. App Stores serve a "store window" purpose, where it is easy to search, easy to discover, easy to access new tools/solutions.

What annoys me is not that "this app could've been a webpage". It is that "this app should also have a web version".

TripIt comes to mind as the opposite way: they started as a website only, and quickly the need to have an app was obvious: GPS integration, offline access, contact list for sharing, and more.