Coins have ridges (also called milled or reeded edges) to prevent a practice called "coin clipping," where dishonest people would shave small amounts of precious metal (like gold or silver) from the edges of coins to collect and use the shavings while still spending the clipped coin at full value. The ridges make it easy to detect if a coin has been tampered with by shaving off edges, protecting the coin’s value and preventing fraud. Although modern coins are no longer made of precious metals, the ridges remain as a tradition and also help deter counterfeiting.
For a joke on why coins have ridges, here is a humorous take: "Why do coins have ridges? So sneaky people can’t slip off their edge without anyone noticing!"
