At the hotel where I am currently employed (as in most if not all in South Florida) there is a 20% gratuity included in every in-room-dining check as well as in every restaurant check. There is also a space encouraging guests to write in an additional tip for the service rendered. There are many generous and grateful guests who do include an additional tip. For them, we are very grateful.
There are many other guests, however, that tend not to tip extra. I'm sure they have their reasons. Many guests are under the assumption that the 20% gratuity included in the check goes directly to the server (or waiter) who serviced or delivered their meal. That is most certainly not the case. Guests don't realize that the 20% gratuity is divided among all the employees on that shift: the servers, the captain, the busser/runner and the order taker.
I suppose that the 20% gratuity is added to each check to ensure that the staff receives some type of compensation for their work since their hourly wage is way below the national hourly minimum wage. There are cheap guests that, even if there was no gratuity included in the check, would still not tip. Thankfully, those types of guests are not the majority.