Lets see. You played Dragon Age Origins? Or know how the Darkspawn work? To reproduce they take one of the civilized races, rape them and corrupt them with blight until they become one of them.
Now Night Trolls work similary. They kidnap a civilized race, they imprison them in their lair. Then through undisclosed, yet certainly horrifying means (its magic, they aint gotta explain shit) they transform the captive into one of them and they become a spouse or husband (gender is maintained, night trolls only kidnap members of the opposite gender). Afterwards they proceed to have kids, which do the same, and the night troll may even get a second, third, fourth, etc. mate should they be successful in their kidnapings.
Basically they are the boogeymen of "eat your food or the boogeymen will take you". The boogeymen themselves serve a diferent purpose and represent a diferent, but similar part of folklore.
Elf royalty seem to be favorite targets in my experience.
That just did a lot to ding whatever remaining sympathy I had for goblins. lol
Night trolls dont have anything to do with Goblins. They are monsters of the night that live by themselves (if without a mate) or with their family (if with a mate) that attack whos only purpose in life is to attack and torment civiliced races, including Goblins.
And night troll isnt a troll in any sense of the word as they are randomly generated humanoids of which there are many different types (eyeless freak, gloom hag, dusk horror, night witch, shadow man, etc.). They are basically boogeymen, only bigger and not infinite.