The en passant move can be utilised when a pawn is offered the option of being able to capture an adjacent file’s pawn, which has advanced two ranks. However, surely the game of chess would be fine enough with the absence of the en passant rule? Yes, it does indeed help locked up pawn structures to not occur, but I do not entirely understand the genuine purpose of the en passant.
If nothing else, it does help to make similar looking pawn structures, behave similarly independent on which row they occur.
For instance with en-passant: white: pawn h5, black pawns on h6, g7 would be somewhat equal to the position where white: pawn on h4, black pawns on h5, g6.
Without en-passant however black would be able to achieve a protected passed pawn (often a huge advantage in many endgames) in the first situation by simply playing g7-g5 without being in danger of being captured.
Thus not having en-passant would change the evaluation of many endgames and complicate rules somewhat because now you'd have to consider on what row the pawns are as well.