"What is the benefit to the player in having to buy access to end-game content?"
It is more exclusive. What is the benefit to the player in only being able to complete a raid dungeon once a week? Same sort of concept, limiting the number of times a content can be run reduces burn out on the content.
Not saying I think the price is reasonable, but the theory behind putting content behind a money gate... well, it isn't completely awful when you consider the exclusive loot rewards for paying to access the content.
More money for OOO since the content isn't nearly profitable enough to make up for the cost of the key. If you want an actual benefit to players then there isn't one, unless you count the armour being rarer for trophy purposes but since they're crap trophy costumes is all they are.