One reason is to bypass the sandboxing requirement of the App Store. And once you're off the App Store, users won't be nagged to update your application, so yo

One reason is to bypass the sandboxing requirement of the App Store. And once you're off the App Store, users won't be nagged to update your application, so you're responsible for getting your users to update all by yourself. At which point some form of over-the-air update is more ergonomic than making users hit your download page to pull in the latest version.