I just noticed that, on StackOverflow, the URL for a user's profile includes both the numerical ID of the user and the URL-safe version of their username (/users/<user-id>/<username>)
For example, the appropriate path to my account is /users/10963964/katty-t-enby-cat.

When I remove the username portion from the URL, it just redirects me, making it as if I had kept the username. However, when I remove the ID, I get a "Page not found." page.
Additionally, when I click on any of the buttons saying "Summary", "Answers", "Tags", etc... it does not change where the URL points to, it just adds a query parameter (tab) at the end of the URL.

So, my question is: what is the point of putting the username in the URL if /users/<user-id>/ leads to /users/<user-id>/<username> anyways?