In chapter 3, Olson defines unconditional election (the "U" of TULIP) by quoting Loraine Boettner's book The Reformed Doctrine of Predestination:
The Reformed Faith has held to the existence of an eternal, divine decree which, antecedently to any difference or desert in men themselves, separates the human race into two portions and ordains one to everlasting life and the other to everlasting death [hell]. (pp. 43 in Olson)
Olson summarizes unconditional election in this way:
God's predestination of the eternal destinies of individual human beings has nothing whatever to do with their foreseen character or choices. [emphasis his] (pp. 46)
Olson then goes on to describe what he calls "the dark side" of unconditional election, that because salvation and reprobation are unconditional, God could save everyone if he chose, but does not. He gives the Calvinist explanation of why this is so by quoting again from Boettner:
The condemnation of the non-elect is designed primarily to furnish an external exhibition, before men and angels, of God's hatred for sin, or, in other words, it is to be an eternal manifestation of the justice of God.
That certainly is an explanation of the purpose/reason for hell that I needed to digest for a while... I think I've heard Piper say something similar, basically that hell exists to display God's justice for His ultimate glory. Perhaps I'm blind, but I don't see that as very glorious.
C.S. Lewis gives a different explanation of hell in his excellent book, The Great Divorce. In the book, Lewis explains hell not as punishment for sin, but as an option that God mercifully allows. God respects an individual's choice to remain separate from Him. Lewis puts it this way:
There are only two kinds of people in the end: those who say to God, 'Thy will be done,' and those to whom God says, in the end, 'Thy will be done.' All that are in Hell chose it.
I see this issue (and I believe Olson would agree) as the main problem with Calvinism. I've discussed this before, and I'm reminded of Wesley's poem Oh Horrible Decree. Here's my favorite part, which still gives me goose bumps and tugs at tears every time I read it:
Oh Horrible Decree
Worthy of whence it came!
Forgive their hellish blasphemy
Who Charge it on the Lamb.
The righteous God consigned
Them over to their doom,
And sent the Savior of mankind
To damn them from the womb;
To damn for falling short
Of what they could not do
For not believing the report
Of that which was not true.