Maybe it is hotel / brand / country dependent, but I've found the Hyatt mobile app to be great and in many ways better than the Marriott app.

I've experienced the following this year:

1) used the app for mobile key
2) requested full housekeeping and other items (towels, pillows, toiletries, robes, slippers, and even a yoga mat and free weights!) via just the app while I was out.
3). I've ordered room service from the app while I was out and it arrived and waiting for me when I returned.
4) I was able to look at a spa menu and make appointments for treatments / massages.
5) I was able to check my portfolio and charges, which updates real time.
6) I was able to book upgrade with points booking online, which previous required a call
7) utilized the chat features with the hotel before and during my stay with requests. I've so used the chat with Hyatt global for points and night posting issues.