For me, I find myself running Windows in a virtual machine. (I am on a Mac.) By having the virtual machine in a different window, I can full screen the VM, and simply switch quickly between the two machine states. This allows me the full screen feel on Windows, and I find it feels cleaner to switch between the two types of desktops.
Now, for everyday programs, I can use the desktops to organize the way I work. I like to read in full screen, with no distractions such as pictures and other windows. So when learning how to use new tools, I put manuals, and tutorials off to their own screen. I put background music, and other things I need to run in the back ground all on one screen, then I don't have to look at them cluttering my Dock, and they are not in my way on screen. (Here's looking at you Spotify/iTunes.)
Then, when I am doing any type of creative work, Photoshop, Maya, they all need as much real estate they can get. Giving them their own screen makes quite a bit of sense.
Not to put too fine a point on my browser usage, I use tabs like the dickens. Now, I may have 60 tabs open at once, but they don't display well on one window. I break them up into groups depending on what they are for. They then get their own screen, and are more useful to me.
Not everyone will use screens like that, but it does help my workflow, so I do.