This is inaccurate. Unlike IRL in which objects can appear at varying focal distances, requiring the eye muscles to constantly adjust the focal distance of the eye, the two images in a VR headset (one for each eye) remain at a constant focal distance as determined by the lenses.

The 3D illusion in VR comes from stereoscopy. The objects that appear in VR are actually all at the same focal distance.

So why do you need glasses in VR? Just trying to understand.

Because there are lenses in the headset that focus the two images at a set focal distant (between a couple of meters and infinity focus, depending on the headset).

You need glasses for the same reason one would need glasses to see a flat image several meters aways, or to see the stars as the case may be.

Over the years ive found it increasingly difficult to find a way it could be harmful: latest nugget that comes to mind is that daylight is 1000s of times more intense than any indoor light

I don't believe this to be true - don't you need a light field for this to be possible? i.e. what magic leap was doing?

It is obviously saliently true just via direct experience. Your eyes focus at different distances when looking at 3d objects in VR. You can feel it!

I am not a physicist by any means, and I am not sure what a "light field" exactly is in terms of engineering, but as a everyday skeptic I am generally rather wary of anything Magic Leap puts out.

You are confusing optical focus (like a camera lens or the lens in your eye) with stereopsis - the mechanism by which your brain detects depth information through parallax.

The VR headset shows each eye a slightly different image. Objects appear in slightly different locations in each image. When you look at these virtual objects, the relative angle between your eyes changes which creates the illusion of depth. (This is my layman understanding. I am open to correction).

The optical focal distance remains constant, however.

I've followed up and verified that indeed, most all VR displays have a static focal distance. Thank you for taking the time to correct my and others misconception on the topic!

Nah, they have fixed focus (I think at infinity, but that might not be true). You use other depth cues in normal VR systems, but not focal depth.