my mile/minutes goal? Am I going a little slower today? Am I having a fast day?
It's quick and easy. If my goal is xx miles in 30 minutes, I need to maintain xxx mph pace. I'm .25 miles behind at 10 minutes? Ok I need to increase my mph slightly if I want to hit my time benchmark.

It isn't the only tool, or way to track and keep pace, but it can be a useful tool. I'm sure the same applies to a treadmill.