AntimSanskar is done to bid farewell to our loved ones who have passed away. A soul undergoes cycles of births and deaths before attaining salvation and going to heaven. Performing antimsanskar is an act of showing respect to the soul of loved ones and aiding them in the attainment of salvation.
A person might have committed different types of karma through various cycles of birth and death. AntimSanskar is an act of good deed (punya) which balances out the karma of the deceased and detaches it from the earthly sufferings. It helps in breaking free of the endless cycles of creation and destruction which are essential to the sustenance of the illusory universe.