Relationships are hard, except in BoomCMS
Relationships are used to define connections between pages and are used when functionality is built into a template. For example a member of staff's profile might be related to a page of content as the author or contact by adding a relationship to the person profile page in the content page.
Reuse, don't rewrite
If the template uses this functionality then it will pull across content from the related page such as the title and the standfirst, and perhaps a feature image.