You can get a good idea of this with virtual desktop software running on Vive. After trying it myself, I was reminded that I actually love retina displays and feel like an even higher resolution would be nice. Also FOV is major. Until I can get massive FOV and retina+ resolution, I will always choose a regular desktop display.

"16K" is what's needed to get a literally retina display in VR apparently. We're a ways off (though making progress).

Why is any of that a good thing, though? I could already have a lot more screens than I currently do, but the two I have is just fine. What advantage do windows floating in space offer me over a physical monitor (that will make it worth strapping a headset on every time I sit down at my desk)?

Less space required. Not limited by the size of your workspace. Trivially move your workspace. Get more "screenspace" at the click of a button instead of having to spend $$$. Power efficiency.

Think about where this will be in 10-20 years, not where it is now. It will be like putting a pair of glasses on, not a huge headset. Once your workspace can exist in 3 dimensions, new paradigms will emerge for arranging a "desktop" that we can't even envisage right now.

Think how much space you could save in an open plan office! Twice as many seats at each desk. A CFO's dream.

With AR UI won’t need to be constrained to windows either. It can be much more integrated into our environment and tools. Virtual ‘screens’ are a natural but kinda dull progression to that point. Although I think we’re a lot further off from realising comfortable, everyday virtual screens in AR than fifteen years. One of the interesting facets of VR is that we can already start playing with these new ideas.