Since third edition there is no such thing as facing in combat. Character counted to be quite mobile during combat, so never facing one specific direction only.
That never returned in any D&D editions, and neither in Pathfinder editions.

Much more reasonable way is flanking. Target can't properly defend itself against two enemies on opposite side of him. Which gives opponents advantage on attack rolls (in 5e optional rules in particular).
And as such rogues can use it for sneak attacks.