Low refresh rate causes the "screen flicker" that you may have seen. (Or see now, depending on your settings.) Most people can detect flicker up to 70Hz, and many up to even 75Hz. The screen flicker can obviously be annoying, but I have read articles detailing that it may also have health issues which suggest that in addition to eyestrain it might cause headaches, dizziness, heart palpitations, nausea, etc. This isn't surprising since the effects of strobing light are pretty well known and if I recall correctly, have even been used in experiments on prisoner interrogation and brainwashing.