It allows you to assess where you are to start with, and then assess your progress in dealing with the issue at hand. 

It goes from 10, when the pain or distress is unbearable and out of control to 0, where you feel great, happy and pain-free. When dealing with children who might not be able to count, the alternative is to go from 10 - arms wide open, and progressively close to 0 - hands together in front of you. 

It's actually interesting to see how a "slight pain" might be rated by some people as a 5 and others as a 2!

Further Reading :
SUDS on Wiki   
More on SUDS creator Joseph Wolpe