Why do we need to play long tones?
Why do we play them in the lower register of the instrument?
Long tone help us do several things:
Set the embouchure for the day. If we start playing in the upper register we then to get tight and it is hard to relax after setting a tight embouchure.
Playing the long tones in the lower register helps us to set
the embouchure better and play with a more relaxed embouchure.
Playing the long tones helps is to fill the instrument with the right amount of air. After we are a wind instrument.
When we play the long tones we can listen to the tone color (sound) of each note and learn to match the tone color form note to note. A completely balanced tone color is very important.