It was introduced as a masking tool to prevent people from logging your involvement in less than reputable activities. Much like the original intent behind 1337 speak itself.
Similar words for divisions of pr0n that are still in use include:
R@ygold - (pedophylia)
qwerty - (bestiality)
showme - (voyuer)
rorikon/lolikon - (anime child)
momashi - (incest)
BBW - (Big Breasted Women)
"pron" or "pr0n" is
An accidental or deliberate misspelling of the word 'porn'. Due to the common misspelling of the word, it has been accepted and encouraged amongst the online gaming community.
It was also used to get around keyword filtering such as in AOL's parental control.
This allowed all the children of the world to have access to all the porn (or pr0n) that is nessessary for a healthy childhood.