Answer & Explanation: In vitro is used when DNA is synthesized in the lab, outside cells, in microtubes i.e., in an artificial environment outside of the living organism. Making DNA in a test tube (by hand) is very labor-intensive and takes a lot of time, whereas, the use of DNA synthesizers designed by companies allow for the production of DNA molecules in both small or large quantities and in far lesser time it would take to do so by hand, in vitro in test tubes.