360 customer view as an example. If you are feeding a real-time decisioning, support, advertising system then you need all of the customer features accessible a

360 customer view as an example. If you are feeding a real-time decisioning, support, advertising system then you need all of the customer features accessible as fast as humanly possible. Ideally a consistent O(1) time. Or it could be a real-time fraud system where you need every bit of customer data in one payload to feed into an ML model. You can't do any of that if you are having to do multiple joins across federated databases.