It breaks them down and puts them in a place of psychological vulnerability; making it easy to instill new ideals and values that adhere to the authoritative figures of the institution. They are not allowed the normal extracurricular activities of civilians because they are no longer civilians, they have a new identity, which needs to be followed by new desires; the desire to appease those in power. It furthers the ideal that a team is only as strong as it's weakest link; you are no longer who you thought you were, you not an individual, but part of a team. If you want your team to succeed you need to do your best, or face social humiliation and isolation.