Good article worth reading in context with the original poster's question. But it all sounds like a word game to me. Kids will still form their groups exactly the same way they always have, regardless of what words they are or are not allowed to use.
"You can get very possessive friendships, and it is much easier if they share friendships and have a wide range of good friends rather than obsessing too much about who their best friend is."
There should also be a consideration about pair bonding. The concept of a "best friend" may be practice for romantic coupling in the future. In that sense, learning the ins and outs of a dedicated exclusive bond might be why we inherently have best friends in childhood. Best friends learn how to stick together even when they disagree, fight (literally), etc... just like married couples hopefully do (without the childish fighting though!).
Eg: When I was single-digit aged, I played with my nearly-same-aged neighbours every day. But they were not my best friends. My best friend lived quite a ways away, and so we got to play together only occasionally. But we considered each other best friends, and it was a solid bond until we reached our tween age years.
Most behaviours are learned in youth, and it shouldn't be surprising that this would be one of them.
