It is usually said that Lord Narayana took ten avataras, but Srimad Bhagavatam talks of twenty-two avataras of Vishnu. The question arises: why did He take avataras? The Lord Himself answers this question, said M.A. Venkatakrishnan, in a discourse. In the Bhagavad Gita, chapter 4, verse 8, Lord Krishna says that He takes avataras for three reasons — to protect sadhus, to kill the wicked and to establish dharma. Now all three of these seem to be simply saying the same thing. In fact, instead of expressing the same thing in three different ways, the Lord could as well have said that He takes avataras to destroy the wicked. After all, once the wicked have been taken care of, the good are automatically protected, and dharma is also established. So why did Krishna mention three purposes, when all three in essence mean the same?
Assuming that the Lord could have just said that avataras were for destruction of the evil, then the next question would be why He had to come to accomplish this. Would His will not have been sufficient to destroy demons? The Lord is described as avApta samasta kAman — He has no unfulfilled desires, for He can have anything He wants to. He is known for His satya sankalpa — at will He can achieve anything. Why did He not then destroy the wicked without coming down? The answer lies in the definition of the word sadhu. A sadhu is not a one who has the appearance of a mendicant. Sadhus are those who want to worship Lord Narayana, to see Him, to offer flowers to Him and to touch His feet. Now, while destroying the evil ones can easily be done by the Lord, even without His having to step down from His abode, how will He give sadhus the boons they seek? So, He comes to grant them their wish of seeing Him.