Freewill signifies division. That is given the choice of 0 and 1, as a mathematically/statistically expected outcome, some will choose 1 while others will choose 0. The end result is that some will be with God while others won't, it's nothing in between to be half with God and half not.
So as long as freewill is granted to the angels, they will have to divide as a logical result. Angels are made to be servants of God and the degree of freewill given to them is less than that of humans. As a result, their division will thus occur in heaven (simply means a place God currently dwells).
Humans are put to a place outside God's realm (unlike the angels born in heaven where God lives), their division is thus here on earth.
The division are all aimed for the goal of building a New Heaven and New Earth (i.e., an eternity) where God and angels and humans (those who made their way here) to live forever.
It has nothing to do with God's omniscience. God just let go what should happen happens. of course, He can remove Satan right after he's created. However this is not witnessed. That is, to others God just randomly remove certain angels right after they are born for no reason. To let go everything as they should means that Satan will be judged openly with a said standard (God's Law) such that he will be legitimately banned from entering the future eternity we refer to as the New Heaven and New Earth.
Edited by Hawkins, : No reason given.