Simon Whistler explains the various historic and practical reasons why some coins have ridges in an episode of Today I Found Out. The practice dates back to the 1700s when coins were made of material that was actually worth the value of the coin, but it continues through today.
Putting ridges on some coins in America got its start back in the 1700’s. At this time, coins were actually made of materials that were worth what the coin was worth. For example, a half dollar silver coin contained fifty cents worth of silver. Likewise, a $10 gold coin contained $10 worth of gold.