You mean a waiter? Someone has to exist between kitchen and table, to take payment, set out food, remediate order issues, and at fancier restaurants, pair and serve wines, and essentially act as a salesperson for the menu. Why would I want to get up from my table and interrupt my conversation, collect silverware and my food, and bump into other people doing the same thing when I can have someone bring it to me?
Also using an app is just gross, are you going to have people wait for another app to download, install, and possibly create a new account so that they can sell your personal information, when you could just tell someone what you want?