At the time, I was prescribed glasses and advised to wear them all the time. But, of course, since I didn't need them in order to function (because I had a good right eye), I didn't. Moreover, I grew up programming and spent much of my adolescence in front of a screen with uncorrected vision. I was warned that there might be problems due to this later, and ignored them, since everything seemed fine. What's more, my glasses' focal point was unquestionably long-distance and simply added fatigue to computer use that was not present without them.
I first started experiencing occasional bizarre headaches in 2010, one of which landed me in the ER because it was so bad. At the time it was treated as an atypical migraine. But things really came to a crisis point in spring and summer 2012. I started getting brief episodes of vertigo after more than ten minutes at a screen. The problem was particularly bad when going from near to far focus or vice versa; I'd get this "need for speed" effect where it felt like the world was topsy-turvy, rushing past me. It was very disturbing for a person who is deeply uncomfortable with not feeling in control. Anyway, I couldn't work, and spent the entire summer doing the one thing that seemed tolerable -- watching TV shows and movies at close range. I did a little bit of travelling and had an absolutely miserable time, compounded by the dryness of the places I visited. The economic and quality of life impact could not be overstated. I was miserable, anxious, and unproductive. I was lucky to be self-employed and somewhat flexible in how I went about working, but getting through even ten minutes of e-mail was a struggle some days.
Lots of misdiagnoses and rabbit holes along the way; migraines (of which I had no history), sinus problems, allergies, etc. I took nasal steroids, did this, did that.
An optometrist prescribed me near-distance computer glasses alongside my long-distance ones, and this did not immediately solve the problem. I struggled with such symptoms for another year or so before the problem was anything like resolved. Even so, I depend on having two pairs of glasses and switching among them scrupulously depending on what I am doing. I can get by with long-distance glasses for a little while when doing computer work, but it's ultimately too straining. The most dangerous of all is no glasses: my problems come back quite quickly if I do that.
That's what happens when you ignore computer vision issues.
Edit: I've also found that I do better with a single large monitor than two medium to large ones.