It might be a way for them to prioritize your order before others as they see how much money they earn, so actually it's a bidding process disguised as tipping. I'm not sure if it's shown in the backend though, but I have seen things like this in other delivery apps.
A lot of times, that makes the order come slower. A higher tip means the app will pair your order with someone else that didn't tip or tipped smaller, using your money to make up the difference. I consistently get faster deliveries when I tip towards "the average" instead of over tipping
Why would they prioritize you when you've already paid? Wouldn't they first go after the possibility of more tip money on jobs that haven't yet paid it? I mean, if I was a driver, unless there are more high tip jobs than I could handle, I'd take those and fill in with low tip jobs, and I'd deliver the low tip jobs first in a hope of getting an after the fact tip.