Children of the Promise - Genesis 25:16 - 17

Many of us may not know this, but Genesis 25 tells us that besides Ishmael and Isaac, Abraham had six other sons with his concubine, Keturah. Perhaps even more surprising to some, before he died, Abraham sent those six sons away from him and Isaac, just as God instructed him to do with Ishmael. Why would Abraham send 7 sons away in favor of one? What are the implications for us?