It's especially useful when you run into odd scenarios, since customer service will always go an extra mile. I used to have Diamond with Hilton when worked as consultant.

Once I booked a room for 2, and showed up as 3 people, got upgraded with no problem.

Another time I had my hotel rebooked to a different one after I already showed up - (there was nothing particularly bad with the hotel, but I was new to the city & picked location badly).