Why does En passant exist

It just happened

To prevent forcing fully closed positions.

En passant exists to prevent a pawn from avoiding capture by moving two squares on its first move. It helps maintain fair pawn play and board control.

Way back when, pawns weren't allowed to move forward two squares. When this new rule began to be implemented, some people were upset that a pawn on the fifth rank could no longer be used to stymie the advance of an opposing pawn.
The en passant rule was introduced as a compromise.

In chess a long time ago people realized that games were too slow with pawns only moving one square, so on the first move pawns can move twice. But that made a bigger problem. Pawns, in the endgame, can just skip over the action and promote to a queen and then win, so then en passant came to be

Thank you all