It is important because Jesus taught about it more than all other Biblical authors put together. Jesus speaks of "eternal fire and punishment" as the final abode of the angels and human beings who have rejected God (Matthew 25:41,46) He says that those who give into sin will be in danger of the "fire of hell" (Matthew 5:22; 18:8-9.) The word Jesus uses for 'hell' is Gehenna, a valley in which piles of garbage were daily burned as well as the corpses of those without families who could bury them. In Mark 9:43 Jesus speaks of a person going to "hell [gehenna], where 'their worm does not die and the fire is not quenched.' " Jesus is referring to the maggots that live in the corpses on the garbage heap. When all the flesh is consumed, the maggots die. Jesus is saying, however, that the spiritual decomposition of hell never ends, and that is why 'their worm does not die.'
If Jesus, the Lord of Love and Author of Grace spoke about hell more often, and in a more vivid, blood-curdling manner than anyone else, it must be a crucial truth.
You can find the whole article here.
Tim Keller is a reasonable, gracious and thoughtful man, which means that someone like me (who is none of those three) will inevitably disagree with some bits of what he says, while loving other bits. If there is a risk in Tim's approach, it's that he is too attentive to the sociological vagaries of the people that he is trying to address. But Tim Keller's judgement is that, looking at the people he is ministering to in New York, the time is overdue for better and clearer and more pungent preaching about hell.
If he had come to this conclusion simply through surveying his audience, then there would be difficulties; for the time might come when someone in such a situation might decide to tone down speaking of hell in all its terror for the fear of driving your audience deeper into a pit of depression and low self-esteem.
But Keller's basic point—make sure you read the article—is that we teach about hell because Jesus taught about hell. Tim's conclusion:
We must come to grips with the fact that Jesus said more about hell than Daniel, Isaiah, Paul, John, Peter put together. Before we dismiss this, we have to realize we are saying to Jesus, the pre-eminent teacher of love and grace in history, "I am less barbaric than you, Jesus--I am more compassionate and wiser than you." Surely that should give us pause! Indeed, upon reflection, it is because of the doctrine of judgment and hell that Jesus' proclamations of grace and love are so astounding.
Preach hell, and you end up preaching grace. Lose hell, and you lose grace as well.