Pruning is necessary in the natural world if a plant is to remain reproductive and bear fruit. When a farmer prunes a plant he cuts off the unproductive branches in order to make the plant produce more fruit. He removes everything which would hinder the growth of the plant.
Pruning is also necessary in the spiritual world. Spiritual pruning is correction by God. The Bible also calls it chastisement. When God "prunes" He removes from your life everything which would hinder your spiritual growth. This process is necessary if you are to bear spiritual fruit:
Every branch in Me that beareth not fruit, He taketh away; and every branch that beareth fruit, He purgeth it, that it may bring forth more fruit. (John 15:2)
Sometimes you do not reap the benefits of pruning because you blame Satan when God is actually the one bringing circumstances into your life to correct [prune] you. The purpose of God's correction is given in Hosea:
Come, and let us return unto the Lord; for He hath torn, and He will heal us; He hath smitten and He will bind us up. (Hosea 6:1)
The chastisement of pruning results in returning to God. Only by returning to Him will you become spiritually reproductive and bear the fruit of the Holy Spirit.