Typically you pay for a battle pass with some sort of currency that costs real money to acquire. The battle pass isn’t anything on its own, but if you play the game you’ll then unlock experience or whatever with the battle pass, thus unlocking whatever it contains. Often that’s cosmetics for the game, sometimes useable items, and sometimes it’s more of that currency that costs real money.
IMO they suck. Usually they expire at the end of the month, so if something comes up (family emergency, computer died, whatever) you wasted it and paid for nothing. It’s a cheap trick to devalue a player’s purchase, and to try and boost player count in lieu of good gameplay.
But apparently if you completed the battle pass, it would give you enough premium currency to buy the one they out next month. So theoretically you might only have to buy it once if you were diligent in finish the pass before it went away. Now they’ve taken that away because they wanted more money.