They are Master Items, here is the link to manual bit about them https://help.qlik.com/en-US/sense/3.1/Subsystems/Hub/Content/Dimensions/create-master-dimension.htm
They are, I think, the only way one can create Drill Down Groups and as such are very useful and I use them a fair bit like that.
I quote from the manual link :
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 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.
Master Items comes handy for Ad-Hoc or Self service reporting for users.
A developer adds Dimension and Measures to Master Item and users can just drag and drop these Dim or measures and use to create their own charts or tables.
You can add whole chart as Master Visualization, just drag and drop it if you using the chart again and again. Same thing with Master Dim & Measures. You can create repeating Measures or Dim, save as Master, later use it in future in different charts.