Why do they even have a “tip” field is beyond me, you already paid for delivery, what do you tip for??!!! And on top they ask you for a tip amount before the se

Why do they even have a “tip” field is beyond me, you already paid for delivery, what do you tip for??!!! And on top they ask you for a tip amount before the service is even performed. Tip is to reward for extra work that server did, what extra work is there in food delivery service? And how do you measure it before you even got the food??