While in this case plenty of sesterces were actually coined, in general you might ask why the Carolingian Empire (and most of Europe for several more centuries) used *librae* in its accounts or why tribute to Athens was stated in talents, neither of which was ever coined.  Uncoined units of account are a convenience that avoids the need of coins for monetary transactions and that cannot readily be debased.    See David Graeber's *Debt: The First 5000 Years* for an extensive discussion.  Units of account that do not correspond to coins are a normal feature of human civilizations, not something requiring a special explanation.

Also, since the sesterce was a quarter of a denarius, and a farthing is a quarter of the old penny (which was a denarius), a sesterce is literally a farthing.  Since Roman soldiers around 1AD were paid about 900 sesterces a year, that translates to 18s9d, or about $1 a year.  That gives you a vague sense of inflation over the last 2000 years.  In the US Army a private first class with 10 years experience makes about $27000 per year.  On the other hand, you can buy a lot more with $27000 today than with 900 sesterces back then.  For purchasing military labor prices may have gone up 25000x, but for many goods it's more like 500x (with a wide range, obviously).