Lord Vishnu allegorically means a self realized soul that has left mortal frame forever. A pure soul atman in the cosmic journey of life reaches state of Lord Vishnu.
Lord Vishnu always rests on a bed made by Adi Shesha, the multi-headed king of serpents. This represents a fully awakened kundalini shakti! Goddess Lakshmi is shown as a consort of Lord Vishnu, sitting by her side. Spiritually allegorically this means that one reaches state of self realization, god realization after demolishing material tendencies forever.
A subdued Goddess Lakshmi represents subdued material tendencies.
The existence of Lord Vishnu in Mahasagar lying on bed of Adi Shesha means a freed soul atman can venture to any part of cosmos at will. Mahasagar allegorically represents entire cosmos.
Lord Vishnu taking an avatar means all cosmic powers manifesting in human form so that one can cleanse the society of its ills, re-establishes Dharma (righteousness) forever. Lord Vishnu taking an avatar also confirms that an avatar is a deadly combination of a self realized soul coupled with powers of Chanakya. Two eliminate misery of thousands of countrymen and soldiers Chanakya got 100 kings eliminated in one night.
Only a knower of all, primarily wisdom contained in Bhagavad Gita and Upanishads (independent spiritual treatises) can successfully manifest form of an avatar. Anything below would not do.
All the three attributes of God Almighty namely Lord Brahma, Lord Vishnu and Lord Shiva never existed in real life. When society reaches a point of no return advents an avatar! This happens every 3500 to 5000 years. Above 3650 years before advent Lord Krishna in Dwapar Yuga.
As Lord Vishnu is considered maintainer of entire cosmos, it is only Lord Vishnu that can don, manifest form of an avatar.