"In this world, nothing can be said to be certain, except death, taxes, and Clark Kent's glasses are a stupid disguise." - Benjamin Franklin
Okay, so maybe Mr. Electro-Kite never said anything about Clark Kent's spectacles, but that's only because he died nearly 150 years before Jerry Siegel and Joe Shuster created Superman in Action Comics #1. Since then, everyone from comic book fans to hack stand-up comedians have pointed out that putting glasses on the most powerful superhero in the world isn't a superbly crafted camouflage.
Which, of course, makes comic book writers and artists' jobs that much more difficult. Sure, there's a level of fantasy involved in creating a Superman comic book. After all, there's no such thing as Krypton, superheroes, or a successful print newspaper. We accept those things are true when we read a comic; yet for some reason, Kal-El son of Jor-El donning thick-rimmed glasses and trying to pass himself off as a bumbling reporter is one step too far.
Still, creators over the past 75-plus years have risen to the challenge, coming up with different explanations for why this isn't a dumb idea, but in fact works. Here are a few/
The Playground Gambit
The first - and still most accepted reason - is that the glasses aren't stupid, you're stupid. In this scenario, used in everything from movies to comics, there's just no reason to think the guy wearing glasses who works as a news reporter, who looks like Superman, is in fact Superman.
Haven't you met any number of people in your life who look almost exactly like a celebrity? "I get that a lot," they'll say with a laugh, and you don't then spend the rest of your waking hours trying to expose that your new co-worker is, in fact, Taylor Swift trying to pull a "Hannah Montana" on you. You just accept they're not Taylor Swift, because why in the world would world-famous singer Taylor Swift dress in regular person clothes and hang out with you, when they could be off gathering an increasingly large army of gorgeous, skinny, tall women who may or may not be preparing to take over the world?
Same thing with Superman: you accept Clark Kent isn't the most powerful superhero in the world, because why would he be? He's just a regular guy. His glasses are the 10-percent change you need to accept Clark isn't Supes: he's just different enough.
The Nerd Stratagem
A close second/natural extension of the first reason is that the glasses are all part of Superman's clever disguise as a bumbling nerd. Not only can Superman fly, run at super-speeds, shoot fire from his eyeballs, ice from his mouth and hold his breath in space, but he's also the world's greatest actor.
Before you start tweeting #SupersSoWhite, beyond the comics (which can get away with having an artist contort/mess up Superman to uber-nerd levels), you can look no farther than Christopher Reeve's movie performance to see exactly how this worked. Reeve changed the shape of his entire body when he went from pretending to be Clark, to becoming Superman, and it's a genius piece of acting. It helps that in this case, Superman was actually one of the greatest actors on Earth - and the over-sized, constantly slipping down his nose glasses were the final costuming piece that sealed the illusion.
The Magical Spectacles
In the superb Superman: Birthright by Mark Waid and Leinil Francis Yu, Superman's glasses were elevated to near magic levels. In the story, Clark had the most beautiful, piercing blue eyes you'd ever seen due to his Kryptonian heritage. So Ma Kent gave him a pair of glasses that muted the color of his eyes. When he was Clark, it looked like he had regular blue eyes; but when he was Superman, he'd take off the glasses and look literally other-worldly.
Note: this is not actually a thing that happens, your glasses can't change your eye color; but it's still a great comic.
Glasses Of Steel
The most unique explanation for why Superman dons eyewear was in the also excellent comic Superman: Secret Origins by DC Comics mastermind Geoff Johns, a minor footnote in comics history the writer snagged from earlier Silver Age (1956-1970) comics. In the revisionist origin, the glasses actually contain small slivers of kryptonite that kept Superman's heat vision in check.
Which doesn't explain why Lois Lane didn't just tear them off and scream, "a-ha!" but at least they have some utility beyond making Superman the world's first hipster with a glasses affectation.
The "Spider-Man" Theory
There's one other explanation: that young Clark Kent actually did need glasses. Unlike his original origin which found him lifting up his crib over his head, in various more modern incarnations of the character, Superman's abilities have developed slowly. Smallville is one example, and even cut scenes from Man of Steel (which showed up mostly in children's books based on the shooting script that I, a grown man, definitely didn't purchase from the children's section of a book store with my own money and then take home and read not to a child, but to myself) had Clark discovering his powers over time, rather than all at once.
The example that's caused the most speculation, though, was Superman Returns. In the movie, it's implied (or at least some viewers have taken the scenes to imply) that Clark was wearing prescription glasses as a kid, that he ditched later. It's also possible he was just playing with an early iteration of his disguise, but imagine if you will that young Superman needed glasses, and then kept wearing them because they looked cool/kept consistent with his childhood appearance.
Whatever the reason Superman wears glasses, and there are many, we do know one thing will be true when Henry Cavill wears them for the first (extended period of) time in the upcoming Batman V Superman: Dawn of Justice - that 'Bats don't make passes at aliens who wear glasses. So if you want to snag your man, Supes, better take those specs off.