Why Do Some Coins Have Ridges?

Some coins have ridges on them to prevent counterfeiting and clipping. These days, most of our coins are made from fairly cheap metals like zinc and steel. Not too long ago, however, coins were made of valuable monetary metals like gold and silver. Those who were inclined to cheat the system would often be tempted to clip these coins, which involved shaving off the outer layer and keeping it for scrap while redeeming the debased coin at face value. Ridges (known as reeding) on coins were introduced to prevent this by making it evident when an edge had been tampered with. Today, reeding is mostly a decoration, but it still appears on most gold and silver bullion coins for its original purpose. 
Why do Some Coins Have Ridges