According to Skanda Purana :
the first father of Manuja (Humans) is sage Kashyapa, the son of Brahma. Brahma gives Kashyap the duty of creation of the people on earth. Kashyapa had two wives named Aditi and Dhiti. From Aditi the human lineage is created, from Dhiti the serpent lineage is created. Therefore Originally, the serpent and man were of the same gotra (descendant).Even, due to the lack of knowledge in our clan, even if a snake is killed in an unavoidable situation, and the last rites or traditional rites are not performed for the kalebara (unsoul body of a serpent) the manuja dharma (human race) will be cursed and torment the whole clan therefore, there is no progeny and the development is inhibited. Therefore, by performing funeral, by performing Pinda Pradhana for the entire serpent family, results in the liberation of the curse, everyone in the family will be blessed by Ananta Nagaraja. So, Ashela Bali has to be fulfilled.