So, Vishnu is that energy which pervades everything in existence- the living and non-living!
Nothing can be in existence without Vishnu. Neither the living nor the non-living can exist without Vishnu. It is the very divine essence within everything in existence, not just on earth, but in everything in this cosmos!
This cosmic essence, this divinity within everything in existence the “Vishnu” within each one of us, is the very life essence, the soul or atman.
As Lord Vishnu is the very essence within everything, it is his presence that allows any-person or thing to stay in existence.
Because of this, Lord Vishnu is considered as the one who sustains- manages and maintains this universe.
If you are not familiar with the Ashta Laxmi, then you may want to watch: Goddess Lakshmi (giver of wealth and affluence)- Symbolism and Meaning of Ma Laxmi (Lakshmi) and Eight Forms of Goddess Lakshmi (giver of wealth and affluence)- Who is Lakshmi and her eight forms?
If we are to ever see an image-representation of Lord Vishnu, then many a times Goddess Laxmi is shown at the feet of the Lord Vishnu, in humility and servitude.
This is because the energy of wealth and abundance, needs to be utilized for the wellbeing and sustenance of the cosmos and so the energy representation of Goddess Laxmi is shown to be in service to Lord Vishnu, who is the sustainer of this universe.