Uni isn't all academics, they want you to be a rounded individual. Uni can get tough, they want to know you have some sort of hobbies and outlets to turn to - no point taking in students who are all purely academics, get stressed out and end up burning out. Also, unis will have lots of different sports, societies, volunteering opportunities etc that they obviously want students involved in as much as possible, so if you show some extracurricular interests that may suggest you'd be willing to do that and not just work work work.