They don't. They affect the environment. Which in turn affects players. Is just mumbojumbo "dey affect me gameplay..." - if it all was driven by some obscure algo that governed economy and world states, people wouldn't even know.Player actions in any game mode may indirectly affect all players. Affecting other players is not limited to in-the-same-instance PvP.
Players have their input in a somewhat dynamic world - you could argue to cut down extreme outcomes caused by player interaction. But me fiddling with the BGS likely doesn't have any effect on anyone.