SD Cards aren't exactly the most reliable of data storage devices. What happens if you've put hundreds of hours of work into a Raspberry Pi system, it's all on your SD Card, and the card craps out. Have you saved your work? Or maybe you need to move to a larger SD Card because you've run out of space. Or maybe you want to duplicate the card to have additional systems. These tasks are pretty easy, but not intuitively obvious. It'll take some time at the command line, but fortunately the commands are easy.