What exactly is the reason for closing threads to start a new one?
The IT guys will tell you that servers parse, index and distribute the data faster and more efficiently by limiting the thread size. Remember that XenForo (or any forum software) is simply a content management systems (CMS) that pulls the data from a database and presents it to us in a pretty package. Things like forum and thread searches should be faster and, in theory, of higher accuracy. So many admins believe it to better the user experience while reducing server load.