Here is another one of my informative blogs, lol. En Passant is useful because it stops your opponent's Pawn from getting past your Pawn on the 5th file. I believe it was put into play to counteract the rule that a Pawn can move two squares up on it's first move. En Passant can be performed only on the 1st move on which the Pawn moves two squares up. En Passant mean "In Passing" and it can only be done with a Pawn, if you will it is their special attack. En Passant enables you to progress your Pawn and wipe out your opponent's Pawn at the same time. It makes it harder for your opponent to promote to whatever when En Passant is possible. The Pawn is unique because it is the only piece that captures differently than how it moves. Pawn's play a major role in chess, but En Passant is probably the best defensive move a Pawn has to offer. Picture En Passant as two "Soldiers"; one is on the 5th file and the other just charged two squares in the direction of the one on the 5th file, as a last line of defense he kills the other "Soldier" preventing a major catastrophy of the "Soldier" going to the other side choosing to free the Queen which was imprisoned early on in the battle. Your opponent will try to go through but with En Passant he/she will be without avail. En Passant is just another tactic and like all tactics it also has a flaw, I'll demonstrate the flaw with the following diagram. It is good and potentially bad. Overall En Passant is a great defensive tactic.