Why is en passant a thing?

I'm just confused 

Initially pawns could only move 1 square. When the double square pawn moves were introduced, en passant was necessary to compensate.

It used to be that pawns could only move one square on every move. When the rules were changef to all then to move two on their first move, en passant was implemented. It helps to prevent being able to lock up positions, giving the option to take the pawn as if it has moved one square.

Still though it does seem like an illegal move and lets say there are 2 pawns locked and the opponent plays 2 pawns and en passant happens  just don't know if that's a fair loss.

This position is an example. Black to move would win if there were no en passant.

A 2100 rated player is complaining about en passant. How is that anything other than pure trolling?

Yes , its one of the most confusing things in chess, I recommend it to be abolished, all those in favour say aye

Bro I wish en passant didn't exist bro I should've beat an IM but then en pessant existsed and I lost