Navigation (including a main menu, link menu, links in the content and a sitemap) helps the visitor get a sense of what other content can be found in the website. Things that help the visitor get a 'sense of place' on any page include the breadcrumb trail, the url structure, and an indication of the active page in the (hamburger or left) menu showing top and bottom pages.
Content should therefore always be hung in the Drupal structure so that the page gets a proper url and breadcrumb trail.
This always applies to all content types with 'page' in them (base page, landing page, index page, list page) and, in some cases, also to other content types such as manual, scheme, service (if not shown automatically).
Placing pages in the Drupal structure is an action that is performed (on demand) at the central level.
Example
In Drupal, when you hang a page in a menu, you create that sense of place. If you don't hang a page in a menu, the page will have a url structure in the form of 'www.ru.nl/<page-name>', the breadcrumb trail will be 'Home > <page-name>' and there will be no menu on the left side of the page.
An additional reason why this is undesirable is being able to find a page in web statistics. These work based on the url, so by creating a good url structure for all your pages, it is easier to segment your statistics properly. For example, we currently have a view with only data on visitors in the target group of prospective employees, by filtering on pages with the url structure 'starts-with ru.co.uk/working-at'. And pages for current students can be found by filtering on 'starts-with ru.nl/students'.
You can't structure pages with a url like 'ru.nl/<pagename>' that way. That will be a mess.
Read more about the principles to consider when making the choice to hang a page in the menu in the article When to use a menu on the left side?