Lets say you have one database for your application and it has Products table.
Products table contains its ID, barcode, price, qty, reorder qty and other product details
You have two different applications one is web application and one is back office. Web application displays your product details on web site and sell it to end cusotmer.
Backoffice used to manage inventory like adding qty, adding new products, updating products, update selling price, discount
IF you add those functionality in wcf services both application can talk to service and gather required data or do the database operation.
Now you want to introduce affiliate marketing kind of tool for third party website to display your products on their website and increase your revenue. You can not give database access to those third party website instead you will expose endpoints for them using it they will get real time product details.
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