Why are coins cylindrical?

Answer 1

Most but not all coins are circular (and thus cylindrical) because it's a very convenient and efficient shape. Circular coins don't have to be oriented any specific way so they can be handled easily as well as packed, sorted, and counted mechanically. If, for example, a coin were rectangular you'd have to line them up neatly before you could put them in a vending machine or coin-counter, but circular coins can simply be poured into a slot or hopper.

Some countries have made multi-sided coins but they usually have rounded corners so they don't wear through pockets or get caught in machines. The UK has used an interesting variation on this idea by making each edge slightly curved in such a way that the coin's diameter is constant even though it's not circular. This design lets the coins work smoothly in machinery while retaining a distinctive shape.