Buzzing on the mouthpiece has many benefits if done with a systematic and observant approach. Because the mouthpiece offers less resistance than does the instrument, buzzing helps accustom the player to using more air. This greater airflow helps let the lips relax and vibrate more freely, producing a more resonant sound. It also makes the player more reliant on his ear to place pitches, just as a singer does. Further, it aids in developing the most efficient and consistent mouthpiece placement. Finally, perhaps most importantly, mouthpiece buzzing allows the player to develop new and more refined aural/physical habits more easily.
Thompson James - The Buzzing Book 
