App store is a great distribution, not marketing mechanism. You still have to do your own marketing. It's just very easy for people to buy an app by tapping a button. And it's the default. Many people would be suspicious of apps trying to charge them in Safari.
Of course, if you have a non-mobile web app and are offering a mobile companion, web version it's fine, but if you're building a standalone iDevice app that you want to charge for, AppStore is the only viable option.
Of course, if you have a non-mobile web app and are offering a mobile companion, web version it's fine, but if you're building a standalone iDevice app that you want to charge for, AppStore is the only viable option.