It is common knowledge that God incarnates Himself at specified times. The avatara concept and the bhakti cult are not there in the vedas and upamishads. During the Mahabharata times and later periods only these came down to humanity. The purpose for which these incarnations take place is spelt out by the Lord Himself in the Bhagavadgita

yadaa yadaa hi dharmasya glaanirbhavati bhaarata
abhyutthaanamadharmasya tadaatmaanam srijaamyaham
(Gita 4-7 )

"Whenever there is a decline to dharma and the preponderance of adharma, at such times, I incarnate myself , O Arjuna". There is a fundamental difference between birth as humans and the divine Incarnations. Humans take birth by the force of karma or the good and bad deeds committed in their previous births and there is no option left to the individual souls whereas God incarnates out of His own free will with a specific purpose every time.

Man is under the control of Divine maya (ignorance) while God imposes on Himself the same maya as shown by the following shlokas:

ajo'pi sannavyayaatmaa bhootaanaameeshvaro'pi san
prakritim svaamadhishthaaya sambhavaamyaatmamaayayaa
prakritim svaamavashtabhya visrijaami punah punah
bhootagraamamimam kritsnamavasham prakritervashaat
(Gita 4-6 and 9-8 )

" Though I am birthless and indestructible as well as the Lord of all beings , taking control of my prakriti I incarnate myself by my own maya. As all the created beings are under control of prakriti and without freedom, I release these beings again and again according to their karma assuming my own prakriti."

Purpose of incarnations:

It is known that God incarnates Himself for the protection of noble souls, the destruction of the evil forces as well as the establishment of righteous living. This He can very well achieve by His simple wish (sankalpa) , being omnipotent. According to the Bhagavata , the most important purpose of Incarnations is to make available ways of spiritual enlightenment and salvation for humanity. In other words , the main objective of every Incarnation is to bring God into the life of mortals and is not limited by place or time.. The Almighty very much loves to be amidst His creations thus providing an opportunity for His contemporaries to be near Him, love, laugh, touch and share experiences with Him as any other mortal or a close relation. At the same time, He makes sure that the people are shown the right way of living to enable them to elevate themselves to higher planes by observing the example of the God-Incarnate. This He does by conducting Himself as the very personification of selfless love, knowledge and truth. Has He not said in the Gita that generally people have a tendency to copy what the best among them does and follow what this man holds as standard ?

yadyadaacharati shreshthah tattadevetaro janah
sa yat pramaanam kurute lokastadanuvartate
(Gita 3-21)

Thus in a Leelavatara, Lord takes birth much like a mortal according to His sankalpa, thus providing a golden opportunity for humanity to observe and study the sportive activities of the Lord Incarnate and contemplate on them. It may be argued that only those who are fortunate enough to be living at the time of the Incarnations stand to benefit by God's physical proximity. But, the lives and teachings during these Incarnations which are elaborated in our Puranas and Ithihasas , have enough material for generations of mortals to contemplate upon and develop the sentiment of devotion.

Descents of the Lord:

Bhagavata proclaims the glory, actions and teachings of Mahavishnu in His various divine Descents, at the same time laying emphasis to His Descent as Krishna considered as Purnavatara with the six fold majesties of aisvarya (omnipotence), virya (powers), shri (beauty and auspiciousness), yasas (glory), jnana ( omniscience) and vairagya (renunciation). Mahavishnu, the Supreme Non-dual Being is at the same time the impersonal Absolute as well as the transcendental divine Person. He embodies in Himself the entire universe with its inhabitants, indwelling and also pervading it. His four-fold Vyuhavataras, three-fold Gunavataras and 23 Leelavataras (sportive divine Descents) are His transcendental emanations. The ten Leelavataras can be classified either according to the evolutionary ascent of life from sub-human species like water-bound creatures, through animals, human-animal combination , to ultimately as human beings as in the case of Rama and Krishna or on the basis of Bhaga or the Divine Majesties possessed by them. Krishna is the only Purnavatara with all the above six majesties combined.

A question may arise. When we talk about Incarnations it is normally assumed that these divine Descents pertain only to Mahavishnu and not to Lord Siva. It has to be understood that the concepts of Vishnu and Siva have no difference and these are but two aspects of the same divine Entity. This fact is corroborated by Acharya Sankara in his Prasnottara Ratna Malika towards the end of this work. He asks the question kashcha bhagavaan ? ('Who is Bhagavan?' ) and provides the answer ' maheshah shankaranaaraayanaatmaikah' ( Bhagavan is the one entity who is both Sankara and Narayana). In Ramavatara human tendencies are predominant though at several instances the flash of divinity could be perceived. Here God-Incarnate Himself asserts that He considers Himself as a human (aatmaanam maanusham manye) and undergoes all the sufferings, anxieties and emotional upheavals, but at times He shows He is above normal humans. This occasional godliness is required to be exhibited in order that humanity could perceive the difference and could emulate and elevate itself to a higher level. On the other hand, in the incarnation as Krishna there is a predominance of divinity so that He is looked upon as a Purnavatara ( Bhagavaan svayam).

At other times when, perhaps, the time is not yet ripe for Divine intervention, God enables the appearance of highly evolved souls or mahapurushas like Acharyas, Alwars, Nayanmars, men of realization etc., to correct the erring humanity and bring it back to the spiritual mainstream for its deliverance and liberation by their exemplary life and teachings.