The Wikipedia article is good at explaining the rationale. According to it, it was added around the time the two-space opening move for a pawn was added to balance the game.
Oddly, though Wikipedia deletes programming languages like Factor due to non-notability[1], editors get to go nuts on everything chess or Star Trek without recourse. If you want to know more about why things in chess are the way they are, Wikipedia is your pal.
[1]: http://en.wikipedia.org/wiki/Talk:Factor_(programming_langua...