On the 15th of Av, years after the Rebellion was crushed, permission was granted for the Jews to bury their dead: Taanit 31a
In honor of the miracle of the preservation of the bodies of the rebels, and of the granting of permission for their burial, the Rabbis of Yavneh established a fourth blessing, "HaTov VeHaMaytiv," for the Blessing after Meals: Taanit 31a