[MatmaRex]

Most of the non-sketchy reasons are related to iOS Safari being crippled.

For example, I worked on a rich text editor, and we wanted to put a bar with text formatting tools above the touch keyboard. This is not possible in the browser: your webapp cannot measure the keyboard or the remaining available viewport (and the keyboard's size depends on the input method and the iPhone model).

Another example I experienced is when we wanted to have full-screen dialogs with buttons at the bottom. If you do that, then the users have to tap your buttons twice, because the first tap only expands Safari's browser UI, and your buttons near the bottom of the screen only work while that UI is expanded.