The Basics of Judaism: G-d & Torah: The Status of Angels:
Why were angels created?
Apparently G-d wanted to populate both the heavens and the
earth with thinking beings; and in both cases the beings
weren't perfect - they had free will, and were capable of
sin. It's not clear whether any angels fell before man
was created; some of them rebelled because they objected
to man's creation, and others were tempted by human women.
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