Thomas, the ability to sideload apps removes the need to follow stupid Apple rules that have literally ruined the internet over the last 15 years. When you submit an app to the app store, if it's a client for an online service, they not only review the app on its technical merits, they also review the service itself the app is for. As in, they will force you to choose between changing your ToS or not having an iOS app. This also makes building third-party apps for services that you don't control difficult or sometimes impossible.