The reality of Hell makes eternity real to people. Natural man lives in the immediate moment and neglects his future. He is like those who were on the Titanic who when told it was sinking refused to believe it was!
Hell was preached more by the Lord then Heaven was! Most people think that they are good enough to get into heaven ( I know I am a sinner, but I never murdered anyone etc).
When the sinner is shown that if he rejects the gift of salvation (Jn.3:36) that is exactly where he is going, he will fact eternity as a reality, not a vague abstraction.
Hell would be a terrible thing to bring up to an unbeliever if the free offer of salvation wasn't also offered with it!
Finally, Hell should motivate us to witness and send missions since we know what fate awaits them even if they do not. Whitefield would begin to weep after his messages and the people would ask why was he weeping and he said because he knew where they were going if they did not believe, even if they did not!