The purpose of the Contents tab in the page editor is to:

The Contents tab in the page editor shows the structure of the page as an ordered view of its sections , rows, columns, and modules . From that sidebar, HubSpot lets users click individual elements to edit or manage them while keeping the page structure visible in sequence. This means its primary role is to display the content elements as they appear on the page, not to switch into a design-setting view. HubSpot documents this area as the place where the sidebar editor shows an overview of the page’s content hierarchy. HubSpot Knowledge Base