Who are the residents of Swetha Dveepa? Why should we perform oblation for a Prappanna as they attain ascension and directly go to Srivaikuntam?

Agama states many yogis particularly Sanaka-Sanakadis reside at Swetha Dveepa.
Even if one attains Srivaikuntam, we do shraddha for the Bhagavan who resides inside the departed soul.