I would agree that this is an important feature, as some people just a general reservation is fine but others are willing to pay, why wouldn’t I want them to select and pay for it?
In our case, they would be selecting a slot because we are a drive-in movie theater, so we sell tickets, food, and merchandise. To take it a step further, allow the restaurant to upload a layout of the tables/slots so that the user can see the table number to make sure they are selecting the table they want.