"Best" friends??

Ok, here's the thing, I've never understood the term "best" friend. If someone is your friend why is it necessary to label them as the "best"?? Is it because they are the "best" listeners? Is it because they are the "best" in giving advice for support? I think that is an unnecessary label that differentiates people depending on how useful they are to you. Hence, you also treat them differently, whether it's good or bad. I've met people who have had several "best" friends at the same time, but later, they were just forgotten about. Isn't friendship supposed to be mutual? What if you decide to label someone as your "best" friend but that other person doesn't think the same about you. That shit hurts.

At this point I do not know/understand what friendship is even about.