Readers like working to figure out what happens in stories. We feel rewarded for putting the pieces together. Stories should always try to be unpredictable in what happens next. This keeps readers on their toes and curious as to what happens next.
Plot twist are a great way to spice up an ending. Nobody wants a predictable ending.
Well, sort of.
That is to say, the basic goal presented in the beginning must be achieved -and that should be extremely predictable.
How it happens, its effect on the characters, and what it means for the story afterwards, should be different than expected.
Not every story needs a plot twist, and too many writers try too hard resulting in a bad ending that do not feel satisfying or organic.
If you want to throw in a big plot twist, make sure you leave enough clues behind for the audience to go back and fully believe it when viewing a second time.