Why do some categories have ID#'s in URL?

[cmeinck]

I was curious as to why some categories have URL's without an ID, such as:
http://xenforo.com/community/forums/announcements/

while some categories have an ID# in the URL?

http://xenforo.com/community/forums/installation-and-upgrade-problems.23/

 

[Forsaken]

I was curious as to why some categories have URL's without an ID, such as:
http://xenforo.com/community/forums/announcements/

while some categories have an ID# in the URL?

http://xenforo.com/community/forums/installation-and-upgrade-problems.23/

When you create a forum and fill the 'URL Portion' you can remove the ID.

 

[Gary]

My guess is SEO and for URL rewrites. You could potentially have categories with the same name:

Parent Category A
->Sub Cat 1

Parent Category B
->Sub Cat 1

With the rewrite it would only send you to the first Sub Category 1 and never the second one. The category number help to identify it as which is which