Sustaining the long notes in one breath enables conscious control of spending and saving air, ie. breath, as well as the balanced quality of tone. Also, by sustaining the long tones lip muscles strengthen and we gain control over them.
In doing so it is reflexively understood to which extent their activities affect the quality of their tone and sustain longer playing. The positioning of the lips to the mouthpiece and the activity of all lip muscles is called embouchure.
After a period of active practice mentioned muscles become more durable, and it is usually said that a perfomer is in good condition and has a fine embouchure.
Taken from my book