Because your time is too important to go through the process of thinking about and typing the same information more than one time. But you say you have never lost a file in all the years you have been using computers. You should count yourself among the lucky ones. Computers are mechanical. Eventually, one way or the other, they fail.
Viruses infect files.
When these natural and unnatural disasters occur, you want to be prepared. Think of it as redundancy. Think of it as insurance. Think of it as that warm fuzzy feeling. Any way you look at it, having a copy of a file to revert to gives one a much better feeling than waving goodbye to your information when they cart off your hard drive to the dumpster because the heads crashed onto the surface and wiped out years of your work.