Why is off season training so important for my high school athlete?

It's Important because its the time when all the changes could be made to help improve your athlete physically (get stronger, take care injury prevention issues, gain muscle, drop body fat...) without it disrupting the performance during competition due to how the body would change. You can make these changes, but still give the athlete the time to adapt his new strength levels, as well as his technique to his/her new body, and then get him ready for the season with an increase in athleticism, but without a loss in skills,