It is true that one may play a hundred games, and never have to use the En Passant rule. However, the knowledge of it is vital so you would never be caught in a bad surprise, when your opponent exercises this option.
In the diagram on the right, White's pawn is on the fifth rank (horizontal row). The En Passant rule is designed to protect White from Black's pawn "sneaking past" White's pawn by jumping forward two squares (from b7 to b5).Under the En Passant rule, if Black moves the pawn two squares to b5, the White pawn can take it by moving to b6 (just as if Black had moved the pawn one square). This must be done on White's next move. He captures the pawn En Passant, or in passing. The Black pawn on b5 is removed, and White moves its pawn to b6.
This rule was added in the 14th or 15th century when the rule about pawns having the option of initially moving two squares was added. The rationale for the option of two steps forward is to quicken up the gameplay.
Henceforth, En Passant is a logical response, so that a pawn cannot pass by another pawn using the two-square move without the risk of it being captured. It sorta patched up the loophole caused by the two steps option.
And just a few FAQs to clarify:
Must en passant capture be done immediately, or can I wait some turns before doing it? You cannot wait. If you want to capture en passant, it must be done on the next move.
Is it obligatory to capture en passant? No. Like any other capturing move, en passant capture is voluntary.
Is it possible to capture en passant more than once in a game? Yes.
