without outsiders being able to intercept the message

To many outside of the group, a jargon (or sociolect) appears to be for purposes of concealing the content of utterances, but to many within the group that is not a conscious decision. Many times a specialized vocabulary develops because the outside, clear language does not have words to cover concepts. That being said, I think that l337 most probably developed for humorous reasons, and perhaps just a touch of "ain't that a cool hack" wonderment.

Ceci n'est pas un seing.