Why do many groups always stand in specific order?

[Siobhan Halo]

It's primarily because someone said this is how you should stand to look best and with the added benefit of stans finding their favs instantly.

In (G)I-DLE's case, and I'm just guessing here, Soyeon and Yuqi are always in the center as they are the best known members by the general public, Minnie and Soojin get the outside places (and they sometimes swap sides) because they are the tallest. Miyeon probably gets the left side because Yuqi and Miyeon nearly always have lighter hair and putting them apart looks. more pleasing.