This depends on what Restaurants are looking to achieve.
For discovery then apps like Just Eat and Deliveroo are an amazing gateway to reach new customers — but remember many of these aggregator services also take up to 30% in commission.
For retention and repeat transactions, Restaurants should investing in their own branded mobile apps to connect with their customers to drive sales by offering a simpler way to order and to encourage loyalty through incentivised rewards.
You could check out AppInstitute a DIY app builder platform that offers restaurants the ability to create their own branded food ordering app with loyalty features for a low monthly fee.
Hope that helps.