Having a best friend is an essential part of being a person. Sure, you might feel satisfied with a coterie of collected friends, all equal, but you're missing out if you don't have that one person who you can dish with after everyone leaves. It may seem childish to call someone your "best friend," but to me it just reinforces how important they are. It's like having a title like "lord," or "duke." It's special. The confidant, the other half, the bestie.
With a bestie, you can be 100% always yourself and not have to worry about how you're coming across. With a bestie, you don't have to work to be understood. With a bestie, you have a forever plus one. With a bestie, you have the person you call when things are falling apart. With a bestie, you have the perfect person to send memes to. And isn't that their most important function?