According to Ethics in Hindu or in Hinduism when someone born, he bears three debts (Rina) with him, they are Deva rina (Debt towards God) Acharya rina (Debt towards Guru) Pitru rina (Debt towards Ancestors)
Doing tarpanam means we are clearing the pitru rina. as well we are helping our ancestors to merge with divine light. Those who do not perform these rituals regularly will suffer from various physical and mental health problems and also from family related problems. The negative effects of not performing this ritual will reflect on you and your family members too. This is because our ancestors have a claim on us as they gave us birth and brought us up.