They are using discipline in practice and that carries over into the other parts of their lives. From personal experience, during a sports season I manage my time in respect to homework much better than after the season has ended. At the same time sports provide a great release for stress. When the stress of school work starts getting to people it can help to go to practice where they tell you what to do and while you are working your body you can give your mind a rest for a few hours. Or if you are having a fight with your significant other it can help to go out on the tennis court and hit some balls as hard as you can for a few hours.
Humans are creatures who get stressed and having an outlet for that can really be a benefit.
Though this reason seems to die off as we all get older and more reasons like the ones above and just plain old habit seem to take precedence, usually the love is still at the bottom of it all. The reason that little kids strap on the shoulder pads for their first football league or pull on the cleats for their first soccer practice. This is the reason that sports have survived for so many years and the reason that they have fans who never even participate in the actual event except as a spectator. The human love for sports has survived for centuries and probably will for as long as humanity exists.