It's meant to be the replacement of the Zip-method for sharing themes in the section below. See the docs for a very short explanation:
The UI Resource Package module includes the sass files, building blocks, page templates and the widgets.
We have used the UI Resources as a style guide for multiple apps within the same company, to make sure we are providing a consistent UI and to benefit from the available helper classes / building blocks. It is really convenient for setting up new projects, especially for developers without any knowledge of styling.
Be aware that if you are required to add custom styling on top of the UI Resource package to not put these files in the theme folder as they would be overwritten when updating the UI Resource Package Module.
it ruins your project styling if you previously had none selected.