The thing with this trend amongst advanced users is quite simple to comprehend. If you are dying to take a new operating system for a test run without completely switching to it, you can install it alongside the current OSes on the computer. Thus, you don’t have to install it over the one you are already running, then reinstall the old one if you don’t find any reason to keep the new one. Other than this, several other reasons are to utilize boot managers and, implicitly, multiple operating systems.
Simple Windows GUI quickly sets up Grub2Win in seconds.