Until they come out with some official source of what and how the Oaths actually work answers will be speculation and opinion based.
That said in the case of Oath of the Ancients don't think of it as Good versus Evil, since they have removed alignment restrictions and allow a Chaotic paladin to take an Oath which would be anathema to a chaotic character, but I digress.
Think of this as more of Order versus Chaos. A great many Fey are chaotic and the Paladins are described as Lawful (and mostly good) in the source material. So the way I explain this away is that these particular paladins mediate between the Prime and the Feywild and keep intrusions and mischief to a minimum as well as dealing with malevolent Fey such as Hags and Redcaps.
Again, until a source material addresses the issues with the descriptive text versus the class powers and lack of direction on the Paladins we will just have to speculate.
I do know they removed alignment restrictions to make them more "playable" by people that didn't want to play "Lawful Stupid" but I think that was a failure of either the player, the DM or both in understanding what the Paladin was originally and how you could actually play Lawful Good.
You had great power but you had some restrictions (obligatory Spider-Man reference not withstanding). Originally you had to have extraordinary ability scores (you once had to roll your stats before you could choose your class sometimes you could not qualify for the one you might want) they had the most restrictions in the game but were one of the most powerful.