Any extra curricular activity is important. Grades are important but most of those who review applications understand that getting acceptable grades these days is easier then one thinks. With grade inflation you must stand out among your peers. If I were to review two students, one with a GPA of 4.0 (first of all I would be suspect of that) and no athletic endeavors and one with a 3.0 with athletic achievements I would choose the 3.0. College is so much different then High school. When in high school a student has very little in the way of distractions. He is clothed, fed and housed by his parents with no other responsibilities then to get good grades. He or she is spoon fed life and knowledge. The parents enforce this especially in some cultures. There is a good chance for the spoon fed individual to fail in college. The added pressure of self discipline instead of imposed by the parents or teachers, the need to do ones own laundry, pay ones own bills, find ones own meals, the indifference of college professors and the distraction of other students about the dorm, drugs, sex and alcohol availability, and the social obligations one encounters in college can become major distractions and in some case greatly diminish the academic performance of those who have never encountered them.
Those who show life other then the classroom while in high school have a greater potential for living life on their own, avoiding the pitfalls and in most cases are more successful while in college.
I was a veteran when I entered college. My high school GPA was 2.5 if that, but I was involved in sports, embraced life and learned to live on my own. I watched my freshmen year as droves of baby einsteins were destroyed by alcohol, relationship and the pressures of day to day living. Those who managed and graduated college were those who yes, could and knew how to study, accept that they were not the golden child they once were in high school, and those who understood the politics of people. They were people who were allowed to fail as children and young adults in high school. They were the ones who tried many things and explored their young teenage world. Yes, they did have good parents, but parents who did not coddle them but were there to pull them out of the flames if they got too hot.
A student who endeavors to be part of a team shows selfless service and in life after college or high school will continue to show selfless service. Those who worry only about their GPA show selfishness and a self centered personality.
Sports teams as well as band, volunteer groups and any other out of school activity shows great promise in a person. Those who do all that and still have a 4.0 GPA well they are the ones who will be success full. Those that are a 4.0 GPA and that is the extent of their high school achievements well I doubt them. Those will less then a 4.0 GPA and who contribute to society well they will also be a good bet.