Why so many different files for one game

[instigator81]

I have noticed some site will have multiple files for the same game. For example there might be three version that are a bad dump, two more versions with something else wrong, and a version that is good. What is the point in keeping all the versions around? Why not just keep the one good version? I am just trying to figure out why someone would want a bad dump of a game when they could have the good version.

[Jale]

That's up to you which version you want to keep. The most important are the ones with a "[!]" mark, which means it's a good dump. Also, check for:

[!] - Verified good dump
[a] - Alternate to the original dump, often done by another dumping group
[b] - Bad Dump of a ROM that probably doesn't work properly
[C] - Complete
[cr] - Cracked
[E] - European release
[f] - Fixed to run better on emulators or bug fixed
[h] - Hack of a ROM that has possibly an intro added or graphics changed
[J] - Japanese release
[o] - Overdump of a ROM which has too much space allocated for it making it too big
[p] - Pirate rom released without consent or stolen from the original dumping group
[PD] - ROM is in the public domain
[t] - Trained ROM that has a small bit of code that lets you cheat in the game
[T]\[tr] - Translation of a ROM from one language to another
[U] - US release
(M#) - Multilanguage (# of languages)
(###) - Checksum
(**k) - ROM size
ZZZ_ - Unclassified
(Unl) or (U) - Unlicensed
(-) - Unknown year

Codes from Wikipedia.org

[alcoatjez]

This is a discussion that has taken place on most forums at least once.
Many years ago, Cowering (creator of the Goodtools) decided to catalog every single ROM that was out. This included good dumps, but also bad/trained/hacked/etc. dumps. The tool was meant to show you what ROMs you have.
After a while, people started collecting EVERYTHING, including the crap. That's why so many versions are available on the net.
If you only want clean ROMs, check The No-Intro Project.