Why do people try to order their kin? Aren’t they equally all the same person…? (Example: Someone will be like Madoka- MAIN KINTYPE, Pikachu- Secondary… etc.)

I have no idea why people do this, honestly. Its another current ‘tumblr trend’ that I don’t understand. If I make assumptions about it, I will probably put my foot in my mouth, so I will be waiting for someone to message me with their reasons, before I make a guess. (I do have thoughts about it, but they arent very nice)