To help reduce the repetition and developer error, Qlik has introduced a master library where we can store reusable items, such as dimensions, measures, and even whole visualizations. For people experienced with Qlik's other products, such as QlikView, just think, "No more linked objects and storing expressions in variables!"
It is easy to think of library items in a self-service context. Don't get me wrong; ultimately, you will have to decide what will be published – from your data model to the world for their own analysis purposes. Having said that, the secret sauce of this recipe is in saving your own time.
It is a productivity hack that implies, "Automation is to your time what compound interest is to money." While it is not an exact parallel, this is a nice concept to frame the usefulness of time-saving functions in Qlik Sense. The effective use of the library saves time spent on scrolling downfield lists, rewriting expressions over and over, and applying a single change in multiple places.
Once you have saved enough time to eclipse the setup investment, the value of taking this approach can only compound with continuous development.