En passent

Long time ago, pawns could only move one square up. At that time, there was no need for en passant. In order to speed the game up, the new rule was implemented, so now pawn could move 2 squares up from their starting position.
By doing this, the game was a bit disbalanced, it was easier to make a passed pawn, there was a need to allow every pawn to capture an adjacent opponent's pawn.
En passant is performing this function, and that is why it was necessary to implement it along with the initial 2 squares up pawn move.
Why is en passant in the game?