Well, the remote app has a number of features not available with the physical remote alone. It has the ability to call up a full keyboard, which is waaaay better than using the arrow keys on the physical remote to type in a search. The app also allows you to edit your home screen and add and remove channels, as well as broadcast video or music from the phone to the Roku.
I need to clarify my earlier comment. They didn't strip the "off function" from the Express+. The Express+ can't turn off the TV because there is an infra-red signal that goes from the physical remote to the TV, in place of the factory TV remote. The app can't do that because telephones don't have infra-red emitters like remotes (the app runs through the house WiFi). In addition, the power from the TV doesn't go through the Roku Express+, so there's no "kill switch" like there is with a cable box or other home entertainment device. I'm not sure if HMDI can send an off command to a TV. In general, I haven't noticed any features missing from the Express+, other than it doesn't send 4K, which I don't have anyway. It's definitely a lot faster than my Roku 3.
