Being good at sports and other things helps because no one wants to admit a one-track minded robot. The prototypical boy/girl from a comfortable middleclass/upper middleclass family who has the luxury to study 12 hours a day and load up on ECs… very very typical. A poor kid who has to help his single mom take care of his little 6 sisters and brothers while balancing school would be a stronger applicant to most top schools than the stereotypical introvert with a 4.0 who has never faced hardship and never does anything else.
Good grades are important but if you balance it out with sports (and other things that demonstrate broad talent) it demonstrates that you just aren’t some anti-social nerdy person with a lot of free time, and have a more multi-faceted multi-talented personality.
You’ll get in to a lot of really good schools just on grades alone, but the some top schools might see you as flawed for these reasons.