The only case where that my be true is the case where you have a sole proprietor where the owner is the only one who touches the books. As soon as you have a bookkeeper who is not the owner, or multiple owners you do need that kind of depth.
I'd bet the OP intended it for use by sole proprietors like himself, and that he never aimed to build a product for bookkeepers. There's plenty of a market for invoicing software that targets sole proprietors.
A product with as much depth would likely appeal to accountants or those that assist them directly.
