I go every 6 months as I have glaucoma. My eyesight has not changed at all thank goodness. I would hate to lose my eyesight. With glaucoma moat people have no idea they have it. It is too late to go see the doctor when you notice something is wrong with your eyesight. If you go yearly for your check up, any changes in sight could be prevented or picked up early. Surely preventing any loss of sight and the need for glasses is important....but once the damage is done!!! Its the same with any illness, people leave it too late to go to the doctor.
Mine have to be checked annually because my mobility permit calls for a medical and eye testing. Why they don't combine both licence medical and mobility medical into one test I can't fathom as the questions are the same apart from my doctor having to re-affirm that I still require the permanent mobility pass.