The Origin of Retro Viruses

The arcgene that you find in animals is apparently very similar to a viral gene called a gag gen. And again, this is the gene that codes for the construction of this this protein shell, the capsida, that protects the genetic material of a virus and allows to get into a cell and infect it. So why would cells in the brain be doing this? One possibility is that it's a way for neurons to sort of control each other, to exert some kind of pressure or influence on each other. But in this case, a neuron could maybe use arc to send arina to a nearby cell which, upon arrival, would influence which genes are activated within that cell.