Question : Why are we doing pitrukarma? As of our belief aatma will get moksha or will have rebirth. Then is it necessary to do this karma every year ?
Answer : Pitrukarma should be done according to the aachaara in which one has lived. Death is a continuation of life and not a stop. When the aachaara differ, the last rite also will differ. The rites of a Vaidika, a Vaishnava, a Shaiva, a Shaktha, etc will all be different. Different castes are just different aachaaras of Sanathana Dharma. So the rites of each caste will be different from the other.
Sons / daughters should take care of their parents when they are alive. Taking care of them when alive with care & attention is the real Shraadham.
Genes contain the Samskaras of the ancestors. We are genetically connected to our ancestors. So even if they attain moksha or are reborn they remain with us in our genes. The yearly Shraadham harmonizes this connection & will bring peace & prosperity to us.