Often you want a form to morph it's presentation based on who is using the form. (i.e. I am a manager so I need to see the approve button, but for everyone else it should be invisible) To achieve this you need to be able to compare the current user against a role/group membership. Would be good if PowerApps had some concept of roles (ala Infopath) that could be used for this conditional logic.
Alternatively if there was some known pattern published that showed how you could achieve the same result it would be a valuable edition to the documentation & tutorials.