In the doctrine and covenants we read
D&C 38:11 For all flesh is corrupted before me; and the powers of darkness prevail upon the earth, among the children of men, in the presence of all the hosts of heaven—
12 Which causeth silence to reign, and all eternity is pained, and the angels are waiting the great command to reap down the earth, to gather the tares that they may be burned; and, behold, the enemy is combined.
The phrase “which causeth silence to reign” means that the Lord does not speak to the vast majority of the people of the earth. The Holy Ghost does not reveal truth to them.
Men come into the church by the Holy Ghost revealing truth to them. That revelation shows them the path, and by that path they gain the gift of the Holy Ghost.
Was it essential that Satan and his followers were cast out of heaven? Yes, it was critical.
Is it essential that the tares be gathered and burned, thereby casting Satan’s followers out of our mortal sphere of existence?
Yes, it is just as essential here as it was in premortality.