What is the wisdom behind supplicating for the deceased with, "O Allah, wash him with water, snow, and hail," what is the meaning of this supplication, and when is it offered for the deceased?

The aforementioned supplication is recited in the funeral prayer, and it is permissible to supplicate with it for the deceased before or after burial. Washing the deceased with water, snow, and hail is a metaphor for exaggerating the effacement of sins and the attainment of forgiveness. It was also said: they were specified for their swiftness in extinguishing the heat of the torment of the Fire.