Appearance and disappearance of God is for the devotees. Paritranaya sadhunam vinasaya ca duskrtam. God comes for two businesses, to give protection to the devotee, and to kill the demons. So, for killing the demons He does not require to come. He has got immense power. Simply by His indication any person can be killed, there is sufficient power: Durga-devi. But, He comes for His devotee, because His devotee is very much anxious, he is always seeking protection of the Supreme. So, because the devotee will be satisfied by seeing Him, therefore He comes. That is indicated.  Because of which, devotee is always feeling separation, just to give him relief, incarnation of God comes.

From a lecture on SB 1.14.44 by Srila Prabhupada; April 18, 1973