Used as central/default repo for project management, backlog, etc. This repo does not have code; it is only used as a container for issues (and associated labels, milestones, etc.).
For w3s project, we use a github project for project board functionality. Although issues in that board can come from any repo, this is just a default home for issues that may not be appropriate for any specific repo or which we simply haven't found the right home for yet. Plus this is the reference we use for syncing labels and milestones to other repos (so this project should always contain the canonical set of labels and milestones for all our projects).