I've been able to find two in-universe reasons why Clark would be wearing glasses before assuming his Superman identity
1) The original script for "Lois & Clark : The Pilot, part 1" makes it clear that the glasses (along with his suit) are part of a general disguise to conceal his ridiculously buff physique;
THE BATTERED SUITCASE WITH THE INITIALS "C.K." rests on a cement sidewalk in downtown Metropolis.
PULL BACK TO REVEAL
CLARK KENT picking up the suitcase. In his late twenties, Clark has
the wisdom of Solomon in his brown eyes, the power of Samson in his
dark hair. His muscular physique is concealed behind a grey suit,
but his sense of wonder at his new surroundings cannot be hidden, even
behind thick black-rimmed glasses."
2) The other factor in that the post-Crisis Clark Kent is well known to be a fan of the film "To Kill a Mockingbird" starring Gregory Peck. The glasses that the main character wears are extremely similar to those chosen by Clark to form his initial disguise. It's very possible that he was wearing them (and assuming a similar hairstyle) as an homage to his favourite actor.
Out-of-universe, obviously he had to be wearing glasses. In the comic-book series, prior TV shows and films, Clark Kent always wears glasses. Had he turned up without them, audiences would have been immediately confused.