Evil Spirits Were Needed in Heaven For a While

Before the Lord came into the world evil spirits were in parts of heaven. This was not a good thing, but Arcana Coelestia 8054 tells us that before they were cast out (Rev 12:7-9) the Lord used them for an important use.

[B]efore the coming of the Lord the region of heaven into which they who were of the spiritual church would come, was occupied by evils and falsities....

The region where the spiritual heaven was to be, was occupied by those who were in falsity and evil, but who could be kept in some truth and good by external means, especially by means of ideas of eminence and dignity, just as is the case in the world, where they who are in evil and falsity are nevertheless obliged to as it were think and speak truths, and as it were will and do goods, by external means, which are honors and gains. The reason why this region of heaven was then occupied by such spirits, was that good ones were lacking, and they who were of the spiritual church had not as yet been prepared, and yet every place had to be filled by spirits, in order that there might be continuity from the Lord down to man, for without continuity man would have perished. At this day also there are some regions of heaven occupied by such; but they who are there are withheld by a strong force from the doing of evils.

[At] the end of the church the evil are cast down, and the regions they had occupied are given to the good, who in the meantime have been prepared for heaven.