What is the virtue of praying over the deceased in the Grand Mosque of Mecca for both the deceased and the congregants?

The virtue of the funeral prayer in the Grand Mosque for the worshipper falls under the general virtue of prayer therein and the multiplication of its reward. As for the deceased, a greater number of worshippers is better for him, and this place is a strong indicator of their abundance, in addition to the supplication for him therein. There is no other specific virtue mentioned.