The most popular method for making coins, starting around 700 BCE, is hammering them out using dies. When you squash a lump of material, it tends to produce a round disk. Plus, round dies don't have to be aligned, while square or triangular ones do. Unintentional advantages to the round shape include decreased wear on corners, the ease of fitting a portrait or animal on the coin, and not having to align them when stacking for counting. The round shape has carried into the modern era where the ability to roll makes it suitable for payment machines. So, in summary, it was a result of the minting technique of the time and has become tradition due to its practical advantages.