why is en passant a rule?

If it weren't for en passant the double pawn move would change the game. It's there to fix the problem that rule created.

I just think it's cool. Have yet to do it, but it's been done on me a few times. Every time my eyes light up as in....DAAAAAAAAAAAAAAAAAMN THAT WAS COOL!
As @tmkroll alluded to, the "2-squares" option was one of the last rule changes to the game. It was added to speed up the game as more working class people played chess. But to keep the existing opening lines relevant, the en-passant rule was needed.
This may not be 100% accurate, but I use this explanation when teaching new players about the rule, so that they can understand it better within context.

how would it be quite dull when I never even heard of the move until it was done to me. It wasn't dull before...

En passant does show itself on occasion when an opponent is trying to bypass an attacker completely or locking down a position. It has much more relevance in endgames though since it causes the evaluation of many to be changed completely because of the new "move two squares on the first move" of pawns. The problem it created forced the introduction of the En Passant rule so that centuries of endgame theory would not be completely destroyed.
Most beginners will rarely have a game decided by the endgame since tactics decide games most often at the lower levels. Based on this, it is no surprise that they would not have heard of the move.