filesystemwatcher addlistener handles in base workspace

I have a filesystemwatcher that is watching a log file using addlistener. The log file contains the location of other files that will need to be monitored using addlistener. To get the secondary filesystemwatcher's to work correctly with addlistener I had to use assignin to save the listener handle to the base workspace with each secondary listener handle having a different variable name. This appears to work but I am surprised that it is necessary to have all of the handles exist in the workspace. What am I not understanding? I am using R2014b.