I’m coming from vBulletin and they have this feature. My reason is to have a “Service Request” category where a user can request for jobs (post as topic for that matter). I want to control how many replies go to that topic.
Eg: You want to to have your lawn cut so you post a request (a topic). People who wants to do the job would reply with a brief description about their business. I want to limit the amount of response to that topic. If not possible then I would have to manually close that topic when it reaches x amount of replies but would love an automatic feature.