Well if you happen to look in the Appendix A you will find the Tale of Aragorn and Arwen. It sets the back drop or backgroud for how they met, fell in love, and how there relationship ended. It also has lot of back story for Aragorn.
I agree Liv Tyler did spectacular job potraying Arwen. I think some of her best scenes are The Flight At the Ford, even though as you already know it was suppose to be Glorfindel. However I accept this change for the movies knowing that you can only have so many characters before you lose the audiance.
Plus I just always love seeing more of Arwen anyway.