Assuming the restaurants margin's are large enough to take the hit, delivery service's provide access to a perceived new market - people who wouldn't would have visited the restaurant in person, but would like a delivery.

Since a customer pays the same usual prices (+ a delivery fee), the brand has the same value on their next in-person visit.

The services are cut throat, they push for massive %'s and expect the retailer to push a 'Get delivery through xxx' message. I know of one service with an EPOS integration, they must be waking up to it now their business is proven.