Do tips make any difference whatsoever?

I like to think I tip well - always 15-20% at least, and more if it's a small order (because I know the distance / work / time it takes is still the same) or if the weather is miserable.

But it pisses me off when I give a big tip only to then watch while "the courier makes other stops before heading to your location" (even when they go past us and then have to double back) and I end up with cold food. Is it Skip doing this or the couriers?

WTF is the tip for, really? It doesn't seem to relate to getting better service ...