Playing brass instruments is an athletic activity. Proper breathing, development of muscles, and physical endurance must be attained to play well, and to survive. 
A football player or tennis player wouldn't step onto the field or court without having built up his or her skills, developed the proper muscles to play their game, or practiced their sport enough to make it even feasible to win.
If you, as a trumpet or brass player, haven't learned how to breath properly, your sound will suffer. You will have either a wavering sound and pitch, a weak thin sound, or will overblow creating a sound that sounds unnatural and ugly.
If you don't have proper development of your lip, finger , and arm muscles, you will simply not make it as a player. Your physical endurance depends on developing these muscles. If your muscles
are weak, you won't enjoy playing and your sound will be thin. If you get fatigued, your lips will simply stop buzzing and you won't be able to play until you've rested. An unpleasant side effect is feeling like you've been punched in the mouth.
Your technical development is also important. Trumpet players have high demands in many musical arenas. Orchestral, concert, jazz, rock, blues, funk, and many, many more. One of the nicest things about playing the trumpet is its diversity. But, that is also its challenge. Being able to handle any technical passage that is thrown at you will make you a commodity.
* My students are expected to practice.