One particular type of counterfeiting involved shaving or "clipping" the edges of coins to acquire precious metal for creating counterfeit coins. A countermeasure, still seen on coins today (for example the 1 euro coin and the American quarter) was a ridged side of the coins. If a coin was clipped, the ridge pattern would be missing. Now, why does that matter? In Rome (and in the United states until the 1970's) the value of the coin came from its precious metal. A person who hands you one denarius is handing you around 4.5g of silver at its height around 260BC, debased to a measly 3.4g of silver five hundred years later circa 235AD.

Standardized coinage means one shouldn't have to weigh it out, but a clipped coin is less than what it should be. Removing the ridges lets the recipient know this coin is not worth what it should be.

What happens to a forger? Well, the lex cornelia de falsis set out different penaltis depending on your social status: "deportatio in insulam... for the 'honestiores,' and the mines, crucifixion, or other degrading punishment for the 'humiliores'" (Harpers Dictionary of Classical Antiquities).

Another fun fact? The US quarter has had anywhere from 89 to 119 ridges, and they could vary depending on the mint which produced them! Numismatics is fun!

Sources:

Peck, H.T. (1898) Harper's dictionary of Classical Antiquities. Harper Brothers, New York, 1898. retrieved from Tufts University

William Smith, D.C.L., LL.D.: A Dictionary of Greek and Roman Antiquities. John Murray, London, 1875. retrieved from University of Chicago