Who prays for someone else, gains a lot for himself, as he makes a gesture of love and he grows in that getting at a higher level.
Then he prays for himself. By the fact that he prays he puts himself in connection with God from whom kindness and power come.
Thus praying for someone else permanently you pray for yourself ceaselessly.
How many can do this? Who can have in his mind all the people without cease? Only the perfect one can virtually have all people in his mind.
In any case the prayer of someone for someone else unites him with that one in his mind. Thus the prayer is a means of unification of the people as it is a means of union between them.
Excerpt from Philocaly XI, note 436 st Barsanuphius and John, Spiritual Letters Father Dumitru Stăniloae.