Why do many groups always stand in specific order?

[felidae]

The main reason is because it's easier than thinking "where should I stand now" every time they take photos, especially during on-the-spot pictures like red carpet etc. And from viewer pov, it's easier to notice who is who if the position remains the same.

As for the order, it depends on each group. Based on age, height, position, if the youngest/the shortest/the center stand in center, etc. And for produce 101 and family-produced groups, based on final rank. For example #1 in center, #2 and #3 besides her/him, and so on.