I use it for several things..
First, I use it so I can test my own software on different versions of Windows (or linux), at different display settings, etc.
Second, I use it to test downloaded software so I don't have to install it on my main machine to test it.
Third, I use it to run a sandboxed/isolated machine that I use to manage some financial stuff for my condominium building so that it is isolated from my main pc.
Keep in mind that there are other virtual machine software apps, some are free (VirtualBox, etc.) -- and I make no claim that VmWare is the best (though I suspect it is, with a price to match) -- it's just what I am used to and it has always done well by me. If you are only using such a thing occasionally then one of the free alternative virtual machines would probably suit you well.