When you create and build your visualizations you will come to the point when you realize you have items worth saving to reuse in other visualizations and on other sheets, for your own sake as well as for others using your app. You can save visualizations, dimensions and measures, as master items in the assets panel. When your app gets published these master items will be available to others in a library panel as ready-to-use visualizations, dimensions and measures.
Master items are very useful, you can use, for example, a master dimension in as many of your visualizations as you like and maintain it in just one place. Any updates you make to the master item will be applied everywhere the master item is used.
One of the purposes with creating and maintaining master items is for other users to explore their own ways and directions in the data, on top of what you have provided in the app as pre-made sheets with visualizations. The users will be able to create their own visualizations with your pre-made master dimensions and master measures, for example.