Hey this is Alan jumping in and I will be talking about how marching band works in high school and how it can benefit you in your applications. While I went to a Texas high school and this mainly focuses on the Texas competitions it can still be applied to other states.
Tuba is the best instrument by the way, not biased.
In marching band you learn a show that will then be performed at halftime of your school's football games. Some schools learn a single show throughout the year while others school learn a unique show for each football game. In Texas we compete in UIL competitions going from Region, Area, and State. To make it out of the Region your band must receive a cumulative score of 1 (superior rating) based on the band's choreography, marching technique, and playing skills which can be seen below.
The choreography category focuses on how interesting of a show it is and how well the band did in executing the directors vision for the show. Now being honest, this is partly out of your control as a band member because your show will be entirely up to the directors. What this means is that if a director decides to choreograph a show that is too easy then the band will likely hit an upper limit to how far it can go (happened to me one year where we were perfect but the show was too easy to justify us moving past Area). However, you can still focus on executing their vision by reaching your spot in time, staying in line with the people in front of you, and moving in time.
Note: The judges do not have a copy of your choreography and as such must guess at what you are making. This makes some mistakes easier to hide while others more difficult if the judges can not figure out what you are doing.
Every school has its own show style but the main styles are "Military" (members march in straight lines with uniform step sizes) and "Corps Style" (fluid movement between various shapes with variable step sizes). Each school then has their own marching style which dictates how members are meant to step. The main styles are "Straight leg" (leg stays straight as you march and toes should come out before the knee), "Bent leg" (leg bends slightly with the knee coming out before the toes), and "High Step" (knee comes up at about a 45° angle and rest of the leg comes straight down with the toes pointed). You will be graded on how well the band keeps its marching form and by staying in step with the rest of the band.
The judges will have a copy of your show music and the band will be graded on how well you play and recognize key moments. The band should recognize when to play loud for large moments and when to back away when there is a solo or feature. The band should also not let the marching impede the playing, focus on keeping the top half of the body straight and still to keep from bouncing with the marching. You will always be surprised to hear about all the little details the judges caught in your show.
You must then rank in the top 2-4 bands of your Area (depends on how many bands make it past district) to move onto State contest. Finally at State every band will be ranked from first to last based on the criteria above with more judges present in new locations. Some judges will be in the press box of the stadium while others will be on the field.
What does band do for me?
Simply put, it burns time and builds leadership.
Marching band is time consuming and if there is one thing that admissions love to see it's applicants being productive every hour available to them. At my high school we had practice every morning from 6:00 am to 8:00 am when school started plus an additional hour after school. I can safely say that band was the most time consuming extracurricular I participated in and likely played a key role in getting accepted to Notre Dame.
That is not to say the time went to waste however, there are plenty of opportunities to reach leadership positions in the band and show off your drive to the admissions office. Section leaders, bass/woodwind captains, drum majors, librarians, uniform officers, these are all leadership roles that admissions love to see because they show that you are passionate about something and that you are willing to take charge. Even if you don't reach a single leadership position you can still talk about how you improved as a musician and how as an upperclassman you were able to teach the new members.