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 of interest if not directly relevant, 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).