It's a shame the tv series, Lois and Clark, never definitively addressed it. I mean, with all his super vision abilities, why would he feel the need to cover his eyes?
That there is probably the best answer.
I would have liked to have seen this explored in a flashback to Clark at maybe 10 years of age, and perhaps his x-ray vision or heat vision just starting, and causing him all sorts of problems.
The solution being special spectacles (lead glass and heat resistant glass combo as the lenses and titanium frame) that block his x ray vision and let him know if his eyes are emanating heat. This would serve to help him feel more human and keep those powers contained around others.
This would explain why Clark Kent would already have a long history of always wearing glasses, long before arriving in Metropolis, and why Superman is the disguise for this version of Superman with his eyes having no constraints, along with his choice of costume. This would also serve to explain why nobody Clark has ever known recognises Superman when he comes out of the closet so to speak, to be Clark Kent.
Therefore, Clark Kent equals a constrained who he is and Superman equals an unconstrained what he can do. To me, this is the definitive live action interpretation of Superman. It makes the most sense. In the pilot episode, the spectacles appear to be a natural part of who Clark Kent is.
Maybe Clark's lifetime choice of spectacles wearing could be explored in a new tv mini series called, "Lana and Clark"? Just sayin..