Although we haven’t seen the actual episode, yet, her appearance basically ensures that Elena’s fate won’t be left in limbo at the end of the show.
In the past shows like The Office and many, many more have brought back characters who have left the series, but there’s something particularly fitting about allowing Elena to wake up in the finale of The Vampire Diaries.
That’s the nice thing about getting final season renewals; shows are then able to wrap up the story in a way that is fitting without having to worry about when the show is potentially getting cancelled.