Some people have it set to super-high numbers by accident - Not everyone does it on purpose.
I'm not sure how it works, but I was once playing a random Survival server, and I went into a cave and didn't need torches or anything. I could see perfectly fine, and I even posted a video of it - I didn't know how it happened, I didn't know why it happened, or even when. (In fact, I think the video was on MCSG! If I recall correctly, I stated, within the video, that I had no idea how it happened)
Later on, I went into my Options or something and saw my Brightness was at a super-duper-high number, and it wasn't before. I was able to fix it that way, but it's happened more than once, and I have nooo idea why.
I no longer have that issue, but at the time I didn't know it was Gamma.
I'm just saying this to show that not everyone purposely sets their gamma to a high number, and it can and has happened by accident.
ALSO! I once read somewhere, not 100% sure whether it was in one of the old Lobbies or the Forums, that performance-improving mods (Such as OptiFine, which can sometimes improve your FPS etc.,) AND mods that change optical/visual things, such as Better Animations Mod (Not sure what Better Animations does, but I heard that it ONLY changes things like how it looks when people walk, fly, etc.) and OptiFine (Y'know, the zoom thingy) ARE allowed. This Gamma exploit, glitch, bug, whatever, changes nothing but how you, the player, and anyone possibly viewing your video, see things. It's purely visual. Yes, you can see tripwires in the dark, etc., but OptiFine gives that zoom thingy, which can help you shoot arrows, see names from afar, see what armor someone has, and MUCH more!
**I know I just admitted to "Breaking a rule," (I once had my Gamma set to high, by accident) but hopefully any staff who sees this will understand that it was an accident and I tried to fix it.