For example, I have two items on an invoice that need to be assigned to the same account. It would be much easier to type '14+15' in the debit field and have it automatically calculate '29' instead of pulling out a calculator or performing mental math every time.
Or, I have a credit charges that apply to an account but no separate credit memo. It would be much more convenient to type '199-89' into a field instead of opening a credit memo every time.
Quickbooks allows you to enter simple equations for addition, subtraction, multiplication, and division. It's kind of crazy that one of the most well regarded products in the accounting software market is behind.