a) roll back apps if a new version is broken/undesirable.
b) sideload apps if an app is not available on the app store because they fell out of grace (eg. fortnite).
c) choose a web browser, on iOS you can have any skin you want, as long as the engine is Safari.
d) choose a media player, no hardware decoding on iOS if it is not an Apple-blessed codec and last time I looked, Apple did not allow mkv and I fail to understand why I need to transcode or remux just to please them otherwise it is software decoding.
e) no emulation of other systems allowed or environments that can run code.
Android still allows all of these things.