In addition to their everyday laboratory/scientific uses the others have already mentioned, I would like to add an additional thought:. . I have read some scientific papers lately which suggest that the human genome can exploit some of the ancient viral genomes found within humans. . . Although most of the viruses of today are thought to be "bad", they certainly do increase the amount of diversity when the human host is able to deal appropriately with their immediate presence. . . Millions of years ago a retrovirus (HERV-W) invaded one of our earliest and best surviving ancestors. All of us today possess this viral genome. . . And it has recently gained attention as being a influential factor in placental implantation. It seems to play some sort of role where embryonic stem cells fuse with other cells during embryo development, and allows the embryo to attach to the placental wall. . . Don't ask me more about this, cause I just posted a question here about this yesterday, hoping people smarter than me will have some answers.. . But, let me add this, when the virus is suppressed, these extremely important functions do not happen. . . In short, viral genomes add greater diversity to our own, sometimes for good, sometimes for bad.