Business logic is often the type code that changes least. If it is well designed, isolated and organized your business logic can survive UI changes, framework changes, stack changes, database vendor changes and all sorts of other "infrastructure" change. If you can't identify that "layer" from the others, then business logic will make its way into all sorts of areas that make change harder. And the ability to change is the very value that software has to offer in the first place. If it didn't need to be resilient to change then we could stick with fixed circuits which are far less expensive to build and maintain.