Why does a dime have ridges?

Answer 1

Because it is traditionally a precious metal coin. The reeding makes it obvious if someone has shaved a bit of silver off.
Prior to 1965, the dime was 90% silver. Ridges, or reeding as it is commonly called, were used to prevent people from shaving off tiny bits of the silver and melting it down and passing off a dime that might only have 9 cents of silver in it as having 10 cents of silver in it. If you look at the denominations that have historically been silver in the US (such as the dime, half dollar, and quarter) you will see that they all have reeding to discourage the clipping of coins. On the other hand, coins like the nickel which (aside from 1942-1945 when silver replaced nickel due to wartime nickel shortages) contain no precious metal, they have a plain edge as does the penny. Prior to the advent of reeding, milled coins might have had a lettered or engraved edge such as we see today on the presidential dollars and the British 1 pound coin.