AFAIK, it’s roughly kinda the same theory we have as to why/how life itself exists. Certain chemical compounds—in the right structure—could potentially spontaneously allow for reproduction, based on the physics/chemistry of those structures. Some of those eventually evolved into what we now are / know as “life”, while viruses are (in some ways) perhaps a more primitive version of that.

It's not about whether they exist or not. They definitely exist.

It's about "what is life?" which is a much, much harder question to answer.

The truth is, it's a separate question. It's easy to define what a virus is, but you can debate all day about what exactly is life, and whether viruses fit into the definition that you land on.

If you're asking what a virus is, it is genetic material, wrapped in protein, that infects other cells, usually very specific cells, repurposing the cell to create copies of the genetic material, wrapped in protein.

When a cell dies, it usually releases its genetic material into the environment. Wild organisms are constantly running across DNA/RNA from dead cells. Some organisms have found that picking up random DNA can be beneficial as it can lead to obtaining genes that will give the organism a special advantage. Imagine this happening millions and millions of times over a period a day. It's not hard to see how some stands of DNA/RNA that make it into cells cause adverse effects and eventually develop into more complex items