Things change. Content gets removed. It gets replaced. It gets updated. All of this can affect the URL structure of the content.
When the URL changes, of course, any links that are set to it will break.
A redirect is what we use to reroute web traffic from an old, or missing, path to the new, correct location. Think of it like a Detour sign in a construction zone.
In Drupal, we use the Redirect module to help us set and manage redirects. This module also provides us some automated functionality that will automatically redirect content when the path changes. If additional work is needed on a path, a permitted user is able to go into the Redirect module's settings and adjust things as needed.