And why do they have different descriptions?
the same reason they added dark and fairy typing. dark was to nerf the overpowered psychic type. it worked too. several generations later we have a fairy type which can pawn both now overpowered dragons and dark types. same with priority. its to much of a staple to not give a few Pokemon better defence (or worse offence) for priority moves.
I don't think there's a good reason for it. Some are just for a gimmick. Like queenly majesty. No real reason for it, just to go along with the Pokemon.
As a few comments mention, the reason why there are abilities that prevent priority moves from happening is to "nerf" them. You, also mention "different descriptions". Basically, these abilities do the same thing as the ability or abilities with the same or similar description, but are under a different name. Like Solgaleo's Full Metal Body ability being fairly similar to Clear Body and White Smoke. Also, like Lunala's Shadow Shield ability being fairly similar to Multiscale.
