The purpose of this folder, and its existance, is directly related to the PrintMonitor utility.
When Background Printing or Desktop Printing are activated, documents to be printed are sent to a file, which resides inside this folder. Before the documents are printed, they reside temporarily inside this folder.
Users of Desktop Printing (instead of Background Printing) will find the folder renamed to Desktop PrintMonitor Documents.
In times of troubleshooting, documents to be printed which have problems can be found inside this folder.
If Background Printing or Desktop Printing are both switched off, this folder is not used.