The reason to close folders is that P8 scans or is supposed to scan all library segments 1 level down from what's open.  If the top level has 3 folders and you leave it open P8 is then scanning whatever number of files is inside the 3 folders whereas if the top level folder is closed it will only scan the 3 folder names.
     If, as an extreme example, you left all folders open it would be scanning every file in the library not just the second level items.