In the book, Glorfindel meets Aragorn and the Hobbits on the way to Rivendell and helps save them from the Black Riders. In fact, he doesn't really do much else in the story.
In the film it is Arwen that meets Aragorn and the Hobbits on the way to Rivendell and helps save them from the Black Riders. After that, she doesn't really do much else in the story except for marry Aragorn at the end of Return of the King. And in the book that is basically all she is there to do.
So, essentially what has happened is that two characters who don't do much outside of their one very specific role in the story have been merged into one character - Arwen.
I think it would have been much better if Glorfindel had been in the story instead of Arwen. The part before Rivendell could have been the same as in the book, and then at the end of Return of the King Aragorn could have married Glorfindel, who he had been in love with all this time but could not hope to marry until he was king of both Gondor and Arnor.
That would have been much better than any of the other pointless and arbitrary changes made to the story structure over the course of the films.
Or they could have just given both Arwen and Glorfindel's roles to Legolas.
Or maybe not.