Hellfire is actually the fire of God's love and glory. It's proof of His presence which can be sensed even in Hell, because He is omnipresent. That's why the inhabitants of Hell feel extreme pain from this fire... because it's the love and glory of God cast down upon these unfortunate souls who have decided to forever hate and despise their Creator. It's proof that God loves all of his Creation, even if they've descended into Hell, and has not stopped loving them. He loves them so much He's willing to give people what they so choose instead of forcing them to go the right way.