Sometimes you get an overpayment from a client, or you have other revenue come in that you don't have an invoice to directly tie it to. Others just find different needs and do different things where a simple IOLTA account isn't enough to get things as accurate as you need.
An "Operating Overage Account" per Matter was designed to meet these kinds of needs and others we might not have seen yet.
A thorough guide on how to configure everything and start using the Operating Overage Account feature is here.