The user group option you saw was likely the global lock / unlock permission setting, which would allow anyone in the user group to open or close any thread on the site. This is different from only allowing users to open or close threads which they have created, which was was a separate option that allowed me to provide that functionality to Premium users on our old forum software.
As
@EF12345678 mentioned, thread locking ability is a rather powerful control which, if abused or used incorrectly, could have significant consequences on the community at large.
Threads only need to be closed under a limited set of circumstances. Just because a thread has not received enough replies or just because someone gets an answer to their question, for example, does not mean that the thread should be closed off for any further clarification or discussion.