The Lord descends to the material world out of His deep compassion for His devotees. There are two reasons for the Lord's appearance or incarnation in the material world. Whenever there is a discrepancy in the discharge of religious principles and there is prominence of irreligion, the Lord descends for the protection of the devotees and the destruction of the nondevotees. When He appears, His main purpose is to give solace to His devotees. He does not have to come Himself to destroy the demons, for He has many agents; even the external energy, maya, has sufficient strength to kill them. But when He comes to show compassion to His devotees, He kills the nondevotees as a matter of course.
- Srila Prabhupada (SB 3.28.29 Purport)