In LDS belief, why are eternal families so important? I know that the family is central to God's plan, and that families give you more joy in life than anything else ever could. But I've often wondered why it is necessary for everyone to be sealed together? 

I know the classroom answers. We teach in the LDS church that families can be together forever, sealed together by the power of the priesthood under the blessings of the temple, but why is that necessary? Why do we need to be sealed to our children and to our parents when in the beginning we were already part of an eternal family; sons and daughters of God.

What is the significance to coming to earth, raising one of our spirit brethren in mortality as our own physical offspring, then sealing them to us as such for time and all eternity? When we die, we're going to remember the pre-existence, and the relationships we had for an eternity as spirit siblings. Am I going to view my daughter as the spirit sister I knew her has? Or am I forever going to regard her as my very own offspring? After exaltation we teach that we begin families of our own, and produce spirit offspring just like our father in heaven, at which point our mortal children will be our spirit siblings, our children from our brief time in mortality, the spirit aunts and uncles to our new spirit children, as well as their brothers and sisters somehow?

What is it about taking our pre-existing family of children of God, and organizing them into lineages for time and all eternity that is so significant?
Eternal families are based in a life in that same organization after the Resurrection and Judgement (cf. Doc &amp; Cov 130:2).

Regarding the resurrection, 


  As part of His eternal plan, God has provided a resurrection for everyone.
  
  
  Everyone who has lived will be resurrected (see 1 Corinthians 15:21–22; Alma 11:41; D&amp;C 29:26; 2 Nephi 9:22).
  Resurrection is the reuniting of the physical body and the spirit after death (see D&amp;C 88:14–17; Alma 11:43; 40:23; 2 Nephi 9:12).
  We cannot receive a fulness of joy when the spirit and the body are separated (see D&amp;C 93:33–34; 45:17; 138:50).
  Resurrection is brought about by the power of God (see John 5:21; Acts 26:8; 1 Corinthians 6:14; 2 Nephi 9:12).
  Resurrection restores every limb and joint to its proper and perfect frame (see Alma 11:43–44; 40:23; 41:2).
  
  
  Doctrines of the Gospel Student Manual; Chapter 32: The Resurrection and the Judgment


While the eternal intelligences inhabit the spirits created by our Eternal Parents (cf. Doc &amp; Cov 93:29 and Abraham 3:21-22), our spirits will, in turn, inhabit the physical bodies which were created for us in this life by our earthly parents (cf. Doc &amp; Cov 88:15).

Our sons and daughters will be organized in a grand family with Adam and Eve at the head of those from this earth.  This grand family is all organized within the Eternal family that includes all of the spirit children of our heavenly parents—including those from the innumerable worlds which God has created. Both lineages are important as they signify the various stages of our eternal progression and through the estates which we have kept (cf. Abraham 3:26).

This lineage is not a "ranking" system of "best to worst," rather it is a system of organization based upon the basic family unit.  Only those in the highest degree of the Celestial Kingdom will be a part of this grand family and are those who are "heirs of God, and joint-heirs with Christ"(Romans 8:17).