Dussehra might be a festival to celebrate the victory of good over evil, but it's only a minor part of Hindu mythology. Ravan was one of the most important characters ever in the scheme of things. He's seen mostly as a villain, but surely he's much more than that.
In one of the many versions of the Ramayana, it is said that once Ram's army had created the bridge to Lanka, they needed to get Shiva's blessing for which they set up a yagya. But the biggest bhakt of Shiva in the entire region was Ravan, and since he was half-brahman, he was also the best qualified to perform the yagya. Displaying honour, Ravan actually showed up, performed the yagya and gave Ram his blessing.
Since Ravan was one of the most learned scholars to have ever lived, Ram asked his brother Lakshman to sit beside the dying demon-king and learn from him important lessons in statecraft and diplomacy.