This Google Developer page details the differences between hosted and packaged apps. In a nutshell, the main difference between them is that hosted apps are the ones that you are thinking of (ones that display a hosted website), and packaged apps are apps that are more self-contained (and include most if not all of their HTML and CSS files, or resources).
On the WrongPlanet forums, there was an interesting page that describes some of the reasons that a developer might make a hosted app. One reason that Apple in my Eye pointed out was:
It may technically be a website, but it functions indistinguishably from a normal non-web-browser app. Also, turning it into a shortcut strips away all the irrelevant browsers controls so that it looks like a normal app window.
NeantHumain continued on the discussion by saying:
So, in other words, it seems that more goes on in the background of a hosted app than just displaying a webpage. It allows the developer to extend the functionality of the website, and bypass some of the restrictions that a normal website may encounter.