Honestly, this a really good and legitimate question. I have wondered that too, but I decided to make my own conclusion for why it exists. I believe it exists because people like to organize things colors, friends, etc. They organize the people they know into zones like "friend zone" to make it easier for them. If I had to say, I believe there are three categories the first and worst is enemy, the second is of course the "friend zone", and last would be significat other or others.