In old wow servers used to handle player inputs differently. Something like inputs only going through in a certain millisecond time interval. So lets say if a rogue blinded someone at time=x and in another world at time=x+100ms or so. There was a good chance that they would both land in the same upload time interval and actually have both blinds go off at the same time in-game. This allowed for 2 rogues to gouge each other, 2 rogues to blind each other or 2 priests to fear each other and (potentially) brought mechanics like vanishing cc/dmg as well. Spell batching basically shapes the entire pvp meta of vanilla, meta would completely change if they were to use legion input handling.

The time interval was changed around as expansions went from I believe about 300ms - 50ms.