What is the need for doing pooja for departed souls?

Jai Guru Deva Datta. In Hinduism, they believe in rebirth, then why they do pooja for departed souls in month of Bhadrapada masam. - Sohan Raju

Rebirth of a departed soul can be immediate or can take any time for which the maximum limit is 360 years (Trai Purusham). This means that the individual soul can stay in the upper world up to a maximum limit of time of three generations. This is the reason for worshipping the departed father; departed grandfather and departed grand grandfather in that month indicated by the three balls of food (Pindas). The fourth generation is not worshipped because the individual soul of the fourth generation must have taken the rebirth since the maximum limit of time is crossed. In the worship of the fore-fathers, people are mistaken in the basic point. They think that they are worshipping the forefathers to get blessings from them. What blessings can be obtained from them, who are also the same individual souls? The basic point in such worship is that you are worshipping the ultimate God to bless them. If you have taken your sick father to a doctor, you must respect the doctor so that the doctor can cure your sick father. Instead, if you start respecting your father neglecting the doctor, can your father cure you when you become sick? You have to approach the same doctor when you also become sick. Therefore, this worship is mainly the worship of God and a little normal respect is given to your fore-fathers in the form of little worship, which is also necessary since they are elders and your well-wishers.