It is easy to understand why Lord Vishnu, descended on Earth taking avatars. To finish evil. But why is it necessary for him to descend on earth? If he comes to earth to save us, that means he is the cause for destruction also. Because he will descend on earth upon his free will? Why can't he finish the earth peacefully? This would, by me, mean that he will descend on earth to prove his ego, his power. Tell me an answer to prove me wrong.
Bhagavan Krishna answers this question:
परित्राणाय साधूनां विनाशाय च दुष्कृताम् ।
धर्मसंस्थापनार्थाय सम्भवामि युगे युगे ॥
To protect Sadhus, and to destroy evil-doers, to well-establish dharma, I appear from time to time.
Yes, he descends out of his free-will to destroy bad people. Yes, he can destroy them just by his sankalpam sitting high up in vaikuntam, and there is no need for him to descend.
But if you notice what he said, killing demons is not his first priority, his first priority is to protect sadhus.
And sadhus are always yearning to see him, meet him, talk to him and play with him. Making them happy is his main goal, and that can be done only if he descends. He doesn't come down for his ego or show off his power, he comes down because his Bhaktas are longing to see him.
While he is here, he also destroys demons and these acts are also enjoyed by sadhus. So Krishna is hitting two mangoes with one stone.
Source - Gita upanyasam by Velukkudi & Krishna Premi. Simply reading bare scripture is not enough to understand true purport of author. Otherwise there is no need for Bhashyams (explanations) by various gurus. If this source is not accepted on this site, answer can be deleted.