I am reading Reclaiming Adoption Dan Cruver, John Piper, Scotty Smith, Richard D. Phillips.  


Here's a great quote that explains clearly and succinctly the Purpose of adoption:


As we will see later in this book, the ultimate purpose of adoption by Christians, therefore, is not to give orphans parents, as important as that is.  It is to place them in a Christian home so that they might be positioned to receive the gospel so that within that family the world might witness a representation of God taking in and genuinely loving the helpless, the hopeless and the despised.