While reasons vary for computer users who desire an open source emulator, common reasons include solving system and disk compatibility issues, system testing, and computer training.
For example: games from the 1990s were meant to play on hardware from that time period. Though Microsoft has kept backwards compatibility, every new system sees its own set of issues (crashes, etc.) when playing Windows 95 games in compatibility mode. Microsoft periodically stops supporting older systems and will eventually phase out the compatibility features with those same older systems. Additionally, as technologies change older computers and their parts become obsolete, thus, harder to obtain. For nostalgic gamers to continue playing their older games, an emulator (virtual machine, etc.) makes perfect sense.
An emulator like VirtualBox will get you a third of the way there. To get the rest of the way, you'll need an ISO of the system you want to emulate and a way to mount the ISO. Of course, this assumes the installation disk is readily available for ISO creation. If the disk is not available but you have your product key, download official Microsoft ISOs here (at last report Microsoft was tweaking which ISOs are available through the download site: msft.digitalrivercontent.net). For more Microsoft ISO options try: https://sites.google.com/site/linuxlablibrary/windows-iso.
PowerISO - recently added a limit of 300 mb for unregistered users trying to create an ISO
IMGburn - DVD burner and ISO creator, be sure to click "custom install" each time it is offered to uncheck the advertisement downloads
*Note: If you like the software, support the teams that created it by donating to their sites.