>>36618141It is due to the diversity of their designs and personalities, before any female role had relapsed into a single pokegirl having to meet all expectations, which gave us a character very much in the style of the Mary Sue, dividing the protagonism allows the writers greater freedom to develop stories and more interesting characters, giving them strengths and weaknesses, that way there is more possibility that the viewer feels empathy for at least one.
This resource is used indiscriminately in harem animes, basically they are proto-characters with pre-established personalities whose success is assured, examples: the tsundere, the kuudere, the yandere, the genki, the himedere/kamidere, the moe, and a lot more of deres.
The difference between their correct and incorrect use is the number of characters you plan to use and the depth they develop throughout the story.