There is a distinction between between a URL Alias and a URL Redirect.  An alias allows users to access a page (node) or file at an alternative path. A redirect is moving a user to the correct file or page (node) because it no longer exists at a previous path. 

Alias lets you choose a nickname for your page, meaning a new URL that points to the content the same way the original URL does.

Learn how to create an alias

When to use an Alias?
When you want visitors to go to "A" by typing in "A" or "B". 

Scenario:

Jackie wants visitors to get to a page she created with the title, "Academic Personnel Office". Drupal will use that title to make a path using "/academic-personnel-office". She likes that path, but she also wants a short path to use, so she creates an alias for this page, using "/personnel". Now that the alias is in place, users can go to that page using either path. 

Redirect moves a visitor to the correct page where the content actually is located. 

Learn how to create a redirect

When to use a Redirect?
When "A" is no longer available and that content is now at "B". 

Scenario:

Jackie had an old site with a page "about.html". Drupal no longer uses the html extension by default. Also, she has moved the content from about.html to a new location, a page within Drupal with a path of "/team". Jackie sets up a redirect so that the old "about.html" page redirects to "/team".