Wikipedia says:
It prevents a pawn from using the two-square advance to pass an adjacent enemy pawn without the risk of being captured.
… which I understand. But why would this be a rule? After all, the pawn that is on the opposing side of the board is closer to being queened. Is the en passant rule to promote offensive attack?
The en passant rule is supposed to minimize the effect of the rule change of the double step.
Basically, people thought it was boring to always make two moves that almost deterministically follow each other, because at the time you always sought to occupy the center, so they combined two moves into one. The same also happened with castling.
But these two moves are still treated like they are single moves made right one after another. Viewed like this, en passant is not actually a rule that was introduced, it was just a logical consequence of making two moves as one.
