Only by choosing the good and experiencing its fruit do we learn to savor and embrace the good.  Similarly, only by choosing the evil and living through its consequences do we learn what evil is and why it deserves to be rejected.  Or as Julian saw, "So pain endures for a time.  Its role is to purge us, make us know ourselves, and it drives us to the Lord."  Ther in lies the "needfulness" of sin, the part it has to play.

Terryl & Fiona Givens, The God Who Weeps, pg. 84