When to use the library instead of templates.
Templates contain the complete structure of a project. When saving a template, there are options to keep the actual values, user and styles of the original project.
When you create a new project from a template, you get a new file with its own Document ID. Just save and enter a new file name. There you go.
So use a template to get easily and free of conflicts complete new projects.
Another case is when you have recurring parts in one project you would like to use in different files. That could be groups with activities and resources.
You drag the groups to the library.
In a different project simply drag the group(s) of your choice back into the outline.
The library is a recycling-kit for different project snippets.