The Projects Tracker list is for managing the high-level details of multiple projects in a single location. It is available in the Project Office template by default, but you can add the list to any other template.
The Projects Tracker list is for projects that you do not want to create a project site for: you only want to track the high-level details such as % Complete and start and end dates. If you want to start assigning and tracking work for one of these projects, you should create a project site instead.