every time your eyes look outside your monitor (even for a fraction of a second), they get a little rest. EG.:Looking a your coffee mug before you grab it, looking at the window, adjusting your keyboard, looking at your colleagues, your 2nd/3rd monitor that's a bit further, moving back and forth on your chair, etc. You never spend 100% of your time starring at the center of your monitor.

On a VR headset, your eyes will always focus at the exact same distance until you remove the headset.