The upside is you can treat it as just another program with a big flat file that serves as it’s hard disk. You can move a VM between computers, they’re universal. Hell you can move it to a data center and hardly notice a difference. You can make a snapshot, try something out, and if it borks, roll it back to a previous snapshot. You can copy the VM any number of times.
Basically it decouples operating systems from hardware so you can treat a computer like software.