According to Vedic scriptures (Puranas) it is very essential to do pitri yagya (yagya for ancestors) for showing respect or to show how obliged one is towards his ancestors for what they did for us.
This yagya helps their souls to rest in peace and we are relived from their debts (our existence is just because of them).
According to Vedic scriptures after the performation of Pitri Paksha yagya the performer will be also bestowed with longevity, knowledge, health and wealth.