The buffets where you HAVE to get a clean plate off the stack by the food for each trip to the steam tables (in the USA, the health codes will require a clean plate for every trip to the buffet to minimize cross contamination issues) - stacking the used plates at the end of the table where the staff can clear them as they come by to refill glasses is a good idea, it keeps the plates that you want to have cleared where they are easily picked up AND you can ask for a refill or more silverware, if you need it (toddlers have been known to drop silverware on the floor) or get more catsup for the fries....or something.
Some buffets, you pay as you go in, some of them you pay as you leave - but the tipping at the buffet is more along the lines of a dollar (or so) a person (more if the toddler leaves a swath of destruction behind them) - depending on how long you were there, the rules at that place (a true self serve place where you get your own drinks, too - there may be a sign asking that NO TIPS be left - or a tip jar at the cash register to avoid the cash being tossed as the staff clears quickly).
Money in the trash isn't always seen & retrieved to be "laundered" (joke - but with some truth to it - bills and coins are amazingly "dirty" if you test them).
If it isn't a buffet - it can depend on if there is enough staff to handle the work load or if someone called in sick (or car break down, weather, etc.) and left them short handed for the day...how much service you get....
An overwhelmed staff won't be refilling your drink, bringing more lemons or catsup, and similar things that add up to better service. Much less picking up plates, asking if you want dessert, paying extra attention to see if you need anything else, or bringing your check to the table quickly.