Kanchi Jagadguru Sri Chandrasekharendra Saraswati Mahaswamigal (20 May 1894 – 8 January 1994) explains the important of avatara of Bhagavan in Hinduism.
The doctrine of the incarnations of Bhagavan – avataras - is another unique feature of Hinduism. The Reality (Sadvastu) is one. That It manifests itself as countless beings is one of our cardinal tenets. It follows that it is this one and only Reality that transforms itself again and again into all those beings that are subject to birth and death. Also it is the same Reality that is manifested as Isvara to protect this world of sentient beings and insentient objects. Unlike humans he is not subject to the law of karma. It is to live out his karma - to experience the fruits of his actions that man is born again and again. But in birth after birth, instead of washing away his old karma, he adds more and more to the mud sticking to him.
Isvara is to be known in different states. That Bhagavan is all - that all is Bhagavan - is a state that we cannot easily comprehend. Then there is a state mentioned in the ‘Vibhuti Yoga chapter" of Gita according to which the Bhagavan dwells in the highest of each category, in the ‘most excellent’ of things.
To create the highest of excellence in human life he sends messengers to earth in the guise of preceptors (acharyas), men of wisdom and enlightenment (jnanis), yogins and devotees. This is another state in which God is to be known. Not satisfied with the previous states, he assumes yet another state: he descends to earth as an avatara. The word ‘avatarana’ itself means ‘descent’. Isvara is ‘paratpara’, that is ‘higher than the highest’, ‘beyond what is beyond everything’. Yet he descends to earth by being born in our midst to re-establish dharma.
If Bhagavan descends to earth again and again it is to lift up man and show him the righteous path. When unrighteousness gains the upper hand and righteousness declines, he descends to earth to destroy unrighteousness and to establish righteousness again - and to protect the virtuous and destroy the wicked. Sri Krishna Paramatman declares so in the Gita.