For years the operating procedure for Seamless/Grubhub/Delivery.com orders was a form that would be faxed to a restaurant, the restaurant would communicate that order to the kitchen (either by it's own POS system, writing the order on a regular order ticket or even giving the kitchen the fax sheet) and then the service would call the restaurant and the employee would enter the confirmation code into the phone.

Some restaurants moved towards having tablets and receiving order via an app, but I'd guess that fax is still an option since it's difficult to integrate with a bunch of different POS systems and there are also a fair amount of restaurants that use hand written tickets to communicate orders to a kitchen.

I wouldn't be surprised if this Fax API is the same technology that those companies use to fax orders to restaurants.