Not only is it true, as you wrote, that most iPhone apps could be websites, but it even happens that some things are available as both and the website is better: Facebook (the app is actually slightly slower and doesn't let you like comments on wall posts, for example) and YouTube come to mind. Note that in both cases the app came out before the highly capable Mobile WebKit-specific version of the site (i.e., the app was the best way to use those services on an iPhone before).

Mobile apps seems like a different scenario though. Those apps are built specifically for a certain hardware/OS combination and are able to utilize the specific technologies it provides to do things far more involved than a mobile app could do. Here, it's literally just code that runs in a browser on a computer, just like a web site, so AFAIK there aren't any great advantages as far as the technology is concerned.

App Store = today's AOL keyword