Maheshwara Puja is offered in the remembrance of a person who is no more to complete any incompletions you might have with them. It is offered so that the departed soul can be liberated in the presence of The Avatar.
Sannyasis have no other desire or intention but to get Enlightened and get dissolved in the Cosmos. So feeding them and offering Puja to them brings about the same effect in the departed soul and helps them attain Enlightenment and get merged with the Cosmos. In this puja the Sannyasis are treated as Lord Shiva Himself and are offered bhiksha and fed.
In the Maheshwara Puja, Swamiji, who is the embodiment of Cosmic energy, personally receives the Bhiksha or food offering and he liberates the departed soul . No matter how many births the person would have taken after leaving the body and no matter if the person has met the Master or not in the life, the Master can intervene and make His presence available in the departed soul’s life and lead it to Enlightenment.