The chess piece above is the bishop. Some sets have a cleft in the bishop and others don't. The classic Staunton chess set has the cleft. Redditor BroodyDoggo asked what it is for in the AnarchyChess subreddit. The answers they got were priceless.
Keep doritos in them for a midgame snack. A rook can be used to hold very small amount of salsa.
insert a coin there to power up your bishop
To use as a whistle when the opponent breaks rules
It's for holding the card you can apply to a bishop for bonusses
It's obviously the mouth
If you want to give your date (the chess term for "opponent") your number/note after a sexy match.
It’s a scar from a previous game
Aerodynamics, for when the bishop “Whooshes” across the board
Bottle opener
But the real answer is that it is made to resemble a Catholic bishop's mitre, a headress which has a front and back separated by a cleft. That's when we found out that more people than you'd suspect had never associated the bishop in a chess set with the church hierarchy. As a practical matter, the cleft makes it easier to distinguish the bishop from a pawn.
However, if we go further back into chess history, we find that the piece only became known as the bishop when the game spread to European cultures in the Middle Ages, and it got its name because it looked like a mitre to people familiar with bishops. Before that, it was the elephant, and indeed is still called the elephant in many countries. The Middle Eastern chess sets of the time were highly stylized, and the elephant was not a recognizable shape to Europeans. In some countries, this piece is called the camel. Now you know.
(Image credit: MichaelMaggs)