Getting customers in and out of your restaurant quickly is the key to racking up points and XP (Experience Points).
Your primary strategy throughout the game is to advance in XP (experience points) so that more chefs become available, to earn coins to purchase ingredients and chefs, to earn tools to upgrade your Ingredient and Dish storage areas, and gems so that you can upgrade chef stations. Each time you level up on your XP, new ingredients, dishes and chefs become available.
But all too often, new customers arrive wanting dishes that aren't ready yet and that may take hours to make. Other customers may want dishes you already have available. So instead of letting customers tie up a table, you can send them away by clicking on the "Exit" button that appears in the Status Bar above their table. That definitely works but then you have to wait until the table is prepared for new customers (2-20+ minutes depending on your XP).
Another way to send them off without having to wait until the table is prepared for new customers is to put the table into storage and then put it right back into the restaurant. New customers will immediately be seated, and hopefully they'll order something you already have prepared. If not, send them off too. It's a lot more work to do it this way, but you can triple the number of guests you serve and earn more coins and XP then if you waited.
The other bonus is that by serving more customers, you'll receive more tools and gems. On average, every 10th customer leaves a tool or gem tip. More tools means you'll be able to upgrade your Ingredient or Dish storage areas even sooner.