Essentially the reason why the did it was they needed the money. In order to become Emperor in the Roman Crises you needed the backing of the various legions and all of those demanded large amounts of money. Also like today wars were expensive and as more wars happened the amount of debt that Rome had increased.As a result of these two key factors Emperors would lower the silver content in order to put more money into the country. This lead to rapid inflation and at various points drastic action by some emperors was required to prevent the complete collapse of Roman currency.

Here is a quick summery that you can read if you would like to know more.

http://www2.gcc.edu/dept/econ/ASSC/Papers2010-2011/Botting_inflationmilitary.pdf