I'd say having a best friend is really valuable for children, mainly because it gives them emotional security during tough times. Like in my case, my childhood friend Li Wei helped me overcome my fear of sports competitions just by offering simple encouragement. That kind of support builds confidence in a way that general friendships can't always provide.
That said, I don't think it's essential for every child to have one single best friend - some kids naturally prefer having a few close friends instead. The key thing is having at least one person they can truly rely on when facing challenges at school or at home.