Ownership of burial land is crucial for both familial and political reasons. Sarah would have received a decent burial regardless of whether Abraham had purchased the cave. But Abraham has in mind also and especially on what is needed for future generations, beginning with Isaac. By purchasing a family burial site, Abraham provides as a permanent holding a special place where the ancestors may always "dwell" among the living, a place of memory and filial piety. Indeed, Abraham is arranging his own place of internment and safeguarding it in advance through purchase so that his son will have a secure place to bury him, side by side with Sarah, his wife and Isaac's mother. Isaac will not need to go begging for burial in Hittite sepulchres; Isaac will not need to bargain as Abraham had done; Isaac will know where his loss should lead him: back to his ancestral roots. Even without further instruction, Isaac at the time of his father's death will be directed to reunite his father with his mother and to recognize in their union the wondrous source of his being. He will be moved to recall their deeds. He will be encouraged to try to walk thereafter in their ways... The burial arrangements that Abraham makes today for Sarah (and himself) will speak for him to Isaac after Abraham has died, precisely when Isaac will need to hearken.Even in grief Avraham has his wits about him to recall what his goal in life truly is: to found a great nation. That end goal cannot be forgotten even for one moment, by the end of his life Avraham has developed into the great father that we needed for the new nation.
Leon R. Kass, The Begining of Wisdom: Reading Genesis p.366
Even with the best training and the best teachers in the world, the student (Yitzhak) needs to be receptive to the transmission. After Sara's death, Yitzhak is so grief stricken that Avraham is forced to find ways to help him deal with the pain and move on. This is a two step process: 1. Bury Sara 2. Yitzhak shall be married.
Thus we see that despite the fact that Yitzhak never sees his father again from the time of the Akeidah to Avraham's death, Yitzhak will clearly understand (if he has not understood it already) that Avraham was preparing him to perpetuate the nation.