Why do many coins have tiny ridges (reeding) on their edges?

In the 17th century, coins were made of real gold and silver. Thieves would shave off the edges to collect precious metal dust, a crime known as 'clipping.' Sir Isaac Newton, as Warden of the Mint, championed the use of reeded edges; if the ridges were uneven, you knew the coin was tampered with. Today, they remain as a tactile aid for the blind.