The first importance of Janmashtami is to observe dharma. Since we humans are all imprisoned in samsara, the Lord incarnates in this prison with us, establishes dharma and redeems us from maya. We can only win over maya if we seek the Lord's aid, as did Arjuna during the battle of Mahabharat. As sage Ved Vyas ordains:"Yato Krishnastato dharmaha, Yato dharmastato Jayaha." i.e. Where there is krishna, there is dharma, where there is dharma, there is victory. Since Duryodhana, leader of the Kauravs did not observe dharma, he did not have Krishna. No Krishna, no victory.