Dual-booting is often a great first step for someone who is new to Linux and wants to try it without giving up their Windows installation. (Or, less commonly, someone who wants to use 2 different versions of Windows or 2 different distributions of Linux.)
Switching “cold turkey” to a new operating system (OS) is not an appealing idea for most people because they won’t know how everything works yet in the new OS. This will prove frustrating if they just need to get something done quickly but can’t figure out how to do it, yet there’s no way to go back to the old OS where they know how to get it done.
Dual booting allows you to try a new OS, with the knowledge that you can always go back to the old one if you get stuck or just want to be back in familiar territory.
Here is an excellent set of articles on dual-booting, tailored for beginners, complete with screenshots and illustrations. It covers various combinations of booting Linux-Windows, Linux-Linux, or Windows-Windows.