They do this because they have no reproductive cell machinery of their own to do this. This is why they are debated as being non living entities. Their DNA or RNA (never both) is programmed to hijack the hosts DNA to make more viruses
Both answers above me are good. However i would like to add, that there is the theory that viruses could be the result of genome reduction taken to extreme levels. They may have started out as a normal microbe but got more efficiant and lost most of their genome. In the end you have a Virus. Its much easier to use things that already exist then to make them yourself. You would rather buy a car then to design and build one for yourself from scratch. However there are some viruses who dont need that much help from cells. Pox viruses for example need just a few ribosomes and tRNA and they are happy. Adeno associated virus however is so lazy it needs another virus present in order to replicate.
This is the first genetic identification of an intracellular host factor that contributes to the ability of the virus to copy its genes, says Ahlquist, an HHMI investigator and a UW-Madison professor of molecular virology. Loss or mutation of this gene severely inhibits multiplication of the virus. Finding the gene may help show how viruses manage to selectively amplify their own genes and gather resources at the expense of host cells. Genes similar to the one found in the yeast exist in many other organisms, including humans. Viruses are resource poor, explains Díez. The typical RNA virus has a half-a-dozen genes, and to succeed in life it needs to commandeer resources from the host cell it infects. By using some of the cell's machinery, viruses can copy their genomes and make viral proteins to be assembled into new virus particles which, in turn, go on to infect other cells,
Viruses aren't microbes, they aren't considered living organisms since they don't have a metabolism. Because of this they require a cell-host to multiply. They use the cell's machinery as you said to reproduce and then kill it. The cell-host is perfect since it has all the enzymes and things needed to make the proteins, and copy the DNA or RNA the virus consists of. Hope I helped :) Timothy well said :)
