Why do dimes and quarters have ridges on the their edges?

Why do dimes and quarters have ridges on the their edges?

Many years ago, when precious metals were commonly used in (some) coins, dishonest people would sometimes shave a bit of metal off the edge of a coin. The change in the amount of metal in the coin, and therefore the value of the coin, would not be large enough to notice easily; the cheater could still spend the coin. When the cheater had gathered enough bits of metal, he could trade them in for something of value, getting more for his money (but not legally).