I can provide what I think would be a pretty common example. I work for an online retailer. We have a main retail site, a mobile site, two APIs, and a number of windows services that perform the back end processing that keeps the business chugging. Each of these apps, or clients, tap into a common service layer that in turn uses a common data layer. Layers in this case prevent a massive amount of duplicate code from having to be written and maintained for each app.