One of the glories of the Roman Empire was its coinage. Contrary to popular belief, Roman coins were produced in such abundant numbers that they remain common today. By far and away the greater number reside in private collections and it is often the case that numismatists and collectors are far more knowledgeable about them than archaeologists unless the latter is a specialist.
Coins are important for two reasons.
First, they are individual records of the portrait of emperors and subject matter like records of wars, or historical events, depicted on the reverse. Secondly, they help date the buildings and sites in which they are found. In the third and fourth centuries, coins were increasingly struck at mints across the Empire both officially and by rebel emperors. Coins were not struck regularly. This is very important.