Dear lunarlander,
That's a good question. Indeed, why use virtualization for any reason, on any platform? Why not just run what you want to run natively?
In my case, I need to do a few things at a time, and bought a very fast computer to do that. From many years of experience, I know that Windows behaves badly when it comes to allocating resources, and usually grinds to a halt at the worst possible moment to devote 98% of my processing power to "System Idle Process," which is hiding, say, determing the geometry of a CD that has been there for several hours, or checking to see if my backup and antivirus programs are the best ones possible in Microsoft's view.
Virtualization offers the possibility of isolating one machine's background activity from the high-priority activity of another, and of customizing the resources allocated to each machine, so that the mission-critical VM can be installed relatively free of bloatware, TSR and other background tasks, which are relegated to a smaller machine which is always on, and in which I don't care how long things take to get done (e.g., overnight).
I don't want to "break the security." I just can't have all of my work getting done in isolation from my main data; nor can I replicate all the data separately in every machine's VHD. I'd spend a month synchronizing and manually matching up what got done where to all the other machines, for every day of actual work.
I just need the virtual machines all to access my computer's hard drive at times, just as Microsoft Word, Edge, Photoshop and PowerPoint all access my hard drives now, sometimes concurrently, without creating any problems or security breaches. The problem of linking a VM to external resources that aren't DVD drives or USB ports is a well-known one. I'm looking for the best possible workaround at this point. If that workaround turns out to be using VMware instead, then I'd have to learn a whole new utility. I haven't tried Hyper-V because it is not designed to be available on my computer's factory version of Windows 10, nor in Windows 7, and it is not known to have consistent performance advantages over VirtualBox.
I hope that makes my area of interest somewhat more clear! Cheers.