Many companies are moving away from constantly building applications from scratch, and instead focusing their development on  building a reusable and testable framework to encapsulate their business rules.  Their frameworks are then used to quickly produce custom apps, webservices, components, etc. It is sort of like building a house. First they build a solid foundation, and then build on top of that foundation whatever structure they may need. If you are part of a team about to head down this path, I would strongly recommend a few of my favorite resources, to make sure you get the foundation right the first time.

Early in 2005, Brad Abrams had a 15 part webcast on "Designing .NET Class Libraries". Its now archived on MSDN, but I dug up the link so you don't have to: https://msdn.microsoft.com/netframework/programming/classlibraries/

Soon after the series was complete, he and Krzysztof Cwalina released a book called "Framework Design Guidelines". The book sums up what you might see in the webcast, and if you decide to take a stab at building your own framework, I would consider it required to have on hand.