King side is the preferable castle, however, sometimes a player is unable to castle king side in lue to a rook movement or an enemy bishop blocking the path.
In this event, it is still wise to castle, even if queen side. So after you queen side castle, you simply move your king one square over to guard the unprotected pawn.
Castling queenside also makes it easier to attack a weak enemy king-side flank. Your kingside pawns are free to assist in the attack. Bayonette charge to victory.
One word: Asymmetry
Many games are drawn because the board is symmetrical and one side gaining an extra move is sometimes not enough to prevent both players from playing equally well: attacking and defending without giving the opponent any ground.
However, self-imposed imbalances on the board create tension and it almost guarantees that one side will have some sort of advantage that may result in a win. One example is the Sicilian Defense. Black responds to 1. e4 with c5 to create some asymmetry. Both armies will develop differently and a battle will ensue that will hopefully result in one side winning.
Castling Queenside is another way to create asymmetry. Both sides are free to launch pawns and migrate all of their pieces to the opposing side. There will be a lot of tension and drama. And ultimately, whoever can get to the enemy King first is likely to win. Contrast this to both players castling on the same side: there will be a lot of concentration of pieces on one side, and both players will keep their pawns back to guard the King. You can still win by outmaneuvering your opponent. But if the two of you play equally well, then it's likely to result in a draw.
Sometimes in a tournament you need to win, so you want to create scenarios that are unlikely to end up in a draw.
What is the point/benefit of castling queenside? I can understand if your structure is all messed up kingside and solid queenside.
Otherwise, it seems like it would take longer, plus there's the unprotected pawn on the wall...