Seth Godin has it right on this recent post.

"The power of a number is the effect we saw when they put a number on restaurants (Zagats) and wines (Parker) and gas mileage (the EPA).  People notice a number, and they work to improve it."

Lean practitioners have been quoting Taiichi Ohno for quite some time, who said, "Where there is no standard, there can be no kaizen."  In other words, find something to measure…give it a number…and now you have something to improve upon…a way to know that progress is being made.