One of the important aspects of keeping your Drupal 8 site up-to-date, secure and performing well is to remove unused modules.

There are a number of reasons why you would want to do this:

• Security - Every module should be kept up-to-date.  If you're not using a particular feature, you're just giving yourself more work to do.
• Performance - Drupal is an event-based CMS so at every point in the page-build process, Drupal is checking to see if any module on your site wants to do something.  All of this adds up!
• Site clutter - The "Extend" menu is long enough!  Not to mention any configuration menu items that a module might add.
• Fluff - Some modules just don't belong - Some development modules such as Devel should never be on a production server.  It's always best to remove those completely.