Who do we think Lord Vishnu is? An attribute of God Almighty, Brahman in Hinduism or an absolute reality! Lord Vishnu is part of Hindu Trinity consisting of Brahma, Vishnu and Shiva.
Spiritually, allegorically Lord Vishnu is none other than an enlightened soul, a self realized soul who after leaving mortal frame has permanently liberated from cycle of birth and death forever. This liberated, free soul atman can now venture to any part of cosmos at will.
Lord Vishnu is always depicted lying on Shesha in Mahasagar (Great Ocean) with goddess Lakshmi at his side! Mahasagar represents the entire cosmos and Shesha the kundalini shakti. The character of Lord Vishnu displays a very significant spiritual aspect of life. Commoners, masses indulging in spirituality can reach cosmic end of life, state of Lord Vishnu the moment they overpowered kundalini shakti, activating it fully to reach crown chakra, also called Ajna chakra.
Lord Vishnu is different from Lord Shiva in the respect that Lord Shiva represents an enlightened one, a self realized soul before leaving mortal frame. The coiled serpents in the neck of Shiva represent a fully awakened kundalini shakti. Lord Vishnu lying on bed of Shesha represents a fully awakened one, a pure soul atman freed from bondage of birth and death.
The prime purpose of Lord Krishna incarnating as an avatar as ordained by God is necessitated every 3500 to 5000 years when life comes full circle on mother earth. During this period of 3500 to 5000 years mother earth witnesses both a peak and a trough in human values. When the society reaches a point of no return advents an avatar! The end of an era is marked by a Mahabharata, a battle of unprecedented proportions. Only then the society is cleansed of its ills.
Such a feat could not be accomplished even by an enlightened one, a self realized soul. An avatar is a deadly combination of a self realized soul coupled with powers of Chanakya (the most able administrator in the entire history of mankind). To save lives of millions of soldiers and countrymen Chanakya got 100 kings eliminated in one night.
Put 100 Chanakya in a row and an avatar is born. The Power manifest in an avatar is truly unprecedented. Symbolically every avatar represents power of truth and only truth.