That's a 2 pole breaker. In your panel, there are two busbars, A and B. Each carries 120V with respect to neutral but they are "mirror images" of each other (when voltage on A goes positive, voltage on B goes negative by the same amount and vice versa). So there's 240V between the A and B busbars. 2 pole breakers tap both busbars for 240V circuits so if you look in your panel, anything that uses 240V (stoves, dryers, air conditioners, etc.) are on 2 pole breakers. The reason the two sides are tied together is that if one side is carrying too much current, both should trip. You don't want half of your circuit (one of the two wires) energized.
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