The Woman in White has a very tidy "happily ever after"-style ending. The good are rewarded. The bad are punished: by the novel's end, the two main villains are dead. As a bonus, another highly obnoxious character, Uncle Fairlie, conveniently kicks the bucket, paving the way for the novel's heroes to move into his mansion and inherit lots of land and money.
This might all seem happy and fluffy and a bit trite. But the novel doesn't totally brush aside the traumatic events that occurred earlier in the story. The fact that Walter puts the story down on paper reveals how the harrowing past still impact the survivors… and how important it is to let the world in on the seriously sordid chain of events that comprise The Woman in White.
Also, the fact that Walter lets "Marian end our Story" (3.5.3.15) is also pretty revealing. Marian functions as a sort of co-narrator and as Walter's wingman throughout the novel.
But Walter isn't just giving Marian props by letting her sign off at the story's end. Marian was dramatically affected by the traumatic events of the story, and though she gets a nice ending, it's not a traditionally happy one like Laura's. Marian doesn't get married — no romantic lead swoops in to rescue her at the eleventh hour. Marian serves as a strong reminder of all that happened before we detoured into Happily Ever After Land, and she also challenges conventional notions of what a happy ending is, especially for a female character.