Although not many Christians have, historically, believed in pre-mortal life; it seems to me a fact of primary importance in understanding this - our mortal life.
(Specifically, I believe the standard Mormon doctrine that we were first created as sons and daughters of God - Heavenly Parents - in spirit form; we then chose to incarnate as mortals, to die, and - thanks to the work of Christ - will be resurrected as immortal incarnates; this sequence being a spiritual progression.)
The reasons are many; but one is that it makes sense of the very different mortal lives that humans experience - including the very different times, places and parents into which we are born and live.
Since we all had a pre-mortal spirit life, we all begin mortal life with very different experiences and needs. Thus it makes sense that - across the sweep of history and throughout the world - we each have such different experiences; indeed each person's mortal experience is unique.
The belief in pre-mortal life makes human uniqueness a feature, not a bug; and the divine intention is that each unique life provides what that person most needs on the basis of their pre-mortal experience.
So, in a broad sense, we know why it is that mortal life is diverse; and since God-the-creator is our loving Father, we can be assured that - overall - this world and our own life has been designed and it operated for our eternal benefit.
Note: I should clarify that I believe in the reality of pre-mortal life not from 'logic' but from personal revelation - that is, through direct and experiential knowledge; but having had this insight, I found that a great deal of what had previously confused me 'fell into place'.