Is their any evolutionary benefit to the sneezing reflex when looking at a bright light source, or is it just an evolutionary glitch with no purpose?

It is more about signal leakage between neurones. The optical nerve is closed to the one controlling sneezing so when the optical is highly excited, it can activate the sneezing nerve.

Source : I have a friend with this and he looked it up once

That's the prevailing theory, yes, but as far as I've found there's not empirical evidence to back it up. The theory does make perfect sense to me, though!

Also, as a sufferer, I've noticed it almost always requires sunlight or something with close to the same frequencies. Most artificial lights don't trigger it *for me.
- Artificial lights absolutely trigger it for me. They just have to be bright.
TIL. I've heard a similar thing about certain smells being closely tied with the memory centre in the brain.
- Out of all of our senses, smell is the one that triggers memory the quickest. The olfactory nerves in your nose are extremely sensitive and have a direct line to the long term memory part of the brain

I have no research to back this up, but I think it's just a glitch that isn't detrimental to passing on one's genes, so the trait got to stay around. Not everyone has it.

But if I were to hypothesize an evolutionary benefit, maybe it is a good thing to sneeze out all the fungal spores from your cave once you emerge. But then again, if it was important, shouldn't this also induce coughing?

Closes your eyes when you stare at the sun?

No! I will burn out my retina and there's nothing you and big sunglass can do to convince me otherwise!

I always assumed it was a hold over from a rodent-like ancestor. Stick your nose out of a barrow, and you want to clear it to get a good sniff of the environment.

It's definitely one of those effects that confuses people. If you don't do it, it seems weird as hell. If you do, it seems weird that some people don't get it.

Not every trait has an adaptive significance.

Wait what? I don't recall doing this. Or do you mean squinting instead of actually sneezing?

No, sneezing, as in the photic sneeze reflex. If I look at the sun by accident I'll sneeze.
- Known as the "suntzu"
- Or as my kids started calling it "Daddy is allergic to the sun"
- TIL ! Wow
Only some people get this reflex (me for e.g.). It's an actual sneeze.
Maybe it's a thing that only certain people do, like rolling your tongue. Looking at a bright light doesnt make me sneeze, and until just now I thought it was just an old wives tale, like remedies for curing hiccups. Doesn't work but widely believed.
- Yeah, it's definitely a real thing, it's fairly common even
  
  But if you don't have it, you don't have it, so looking at bright lights isn't going to help you sneeze
Come visit Disney with my family this fall. You'll see me and my kid both sneeze every time we step into the Florida sun after leaving a slightly darkened show build.
- Oh I'd love to, if only I could afford it