This priority gives you the cheapest price you can get out there. Best choice when you're not in rush and your deadline isn't tight. Customers with higher priorities will be served first.
Probably best of both worlds. You get quite a low price and relatively short waiting time in queue. Only High Priority customers are served ahead of you.
Ideal price plan for customers with immediate deadlines. Customers paying for High Priority will always be served first ahead of other customers. In simple words, you pay more to save time.