There are many people who use such kinds of applications because they also get an exclusive discount as well! For example, when you buy burgers, you may get a 10 percent discount. However, you may need to pay some delivery charges, so we have to keep that thing in mind.
Personally, I haven't used any of their services to get my food delivered. This is because I have a very serious trust issues with my food. I can't have an unknown person get me my food and deliver it when I use the app. They night off discount for using their services, I'm not sure.