why is en passant a rule?

It's not random.  If e.p. didn't exist, then as soon as someone got a pawn to the fifth rank, a person could move their neighboring pawn forward two spaces and lock up the position.  Example: you play a2-a4, then a5 and your opponent plays b7-b5.  Now you can't capture that pawn and the position remains closed.  Without e.p. many more games would feature impenetrable pawn walls all over the board and more games would end in draws because people would have to just shuffle pieces back and forth on their side of the "wall".  e.p. allows more captures and more open lines and thus more active play

It was introduced after the rule change to allow pawns to move two squares from their starting position so as to make the game go faster. Since that meant pawns on the 5th rank would lose the opportunity to capture a pawn that originally would have to move 1 square instead of two.

Didn't mean to double post, sorry.

It seems like a very thoughtful balance to the rule that allows pawns to take two squares with their first move.