YMMV (highly dependent on the ambassadors, at least for SPG), but I feel some random quests such as could you request a corner room etc might be fulfilled ahead of time instead of at check-in (also you only need one point of contact instead of dealing with multiple hotel concierges), could you book this hotel for me from XX to XX etc. It was definitely not a game changer for me, probably because my ambassador didn't do too much, not even a birthday email/gift.
Not sure how it will change under Marriott's umbrella.