Because if some of their customers are prepared to keep a reminder of the restaurant on their home screen, why would they turn that down? And it's a lot easier

Because if some of their customers are prepared to keep a reminder of the restaurant on their home screen, why would they turn that down? And it's a lot easier to tell them to install an app which they understand the concept of and have done many times before than to try to explain how to add a website to their home screen.