Why we start with finance
Finance already needs data from almost every other department. Closing a period brings in sales, purchasing, operations, hours and stock. That overview is rare in most companies.
Finance is also where the definitions come together. What counts as revenue, what a customer is, what margin means, when an order is finished. Other departments rightly have a say in that, but financial reporting is where it has to be settled once. And those definitions are the real product of a data model.
So we start at the financial layer. Once the ledger, the entities and the periods are right, adding an operational domain is a small step. Start at operations instead and you are building on numbers that are not settled yet; that layer has to go in underneath later anyway.