This feature is used for when you want to use the exact same page in another project and both projects need to be simultaneously updated when changes are made.
Go to the project where you would like to add an existing page from another project. In the page overview, click the Add page button and choose Linked Page. A menu will pop up where you can choose a project in the drop-down and the desired pages that you want to link by ticking the boxes. Click Add to link the chosen pages to the current project.
It will show the same page in your current project with a lock icon in the thumbnail of the page and 'LINKED SPREAD' description. NOTE: if you want to make changes to the page, only the original page can be edited.
The projects with linked pages need to be published again when there have been changes to the original page. If you don't need this type of functionality, you can just use the duplicate page option.