Sure, but most people's reaction to "This computer has become too slow to do web browsing." is not "Meh, I'll make a VM over in that server that has resources to spare and use that to do my web browsing."
The VM is actually to provide an isolated environment for web browsing. The modern web browser is basically one giant vulnerability. There's nothing going on in the browsing VM other than browsing, so if I go to a dodgy website inadvertently, or get a virus warning, all I need to do is to go up and click "Snapshot->Revert" and it never happened. In like fifteen seconds. The sum total of the damage done is that you've probably lost some recent tabs.
With a PC, you first go "aw, shit," then you figure out whether or not you really want to go and reformat the PC, and then restore it from backup image. Most often you'll choose not to, because, y'know, it's easier to be lazy than to put in work and effort. One of the things I've always been big on in computing is that something that encourages you to do the right thing at a minimal cost is often the ideal solution.