This is used to build a flattened dependency tree for handlebars files.
I mainly use this in component libraries in order to allow white listing of components. For example if you only want to include x-foo from your component library but it is dependent on x-bar which is dependent on x-baz then you need to include all of these files.
This can be done with a static map on smaller or stable libraries such as ember-bootstrap. However for evolving component libraries this can be a source of weird bugs so this solution is advised.