Why is this important?
It allows you to really check your embouchure to see if you're setting yourself up for the best possible tone and tuning you can achieve.
How do you use it?
Ideally, you should check your sound with just your mouthpiece against a tuner. If your sound is too high, you're likely pinching too hard on your mouthpiece/reed or blowing into the tone hole too much (flute). If the sound is too low, more support will produce better sound.
If you're a brass player, you should be able to buzz almost any note your instrument can play. Try using a tuner and seeing if you can play a scale on only your mouthpiece in tune!