Better Utilization of CPU Time
Many times your application isn't really doing any work while it's still enjoying its timeslice. In my document retrieval example, one thread was waiting on the jukebox to load the platter. Obviously, this wait was a hardware issue and required no use of the CPU. Other examples of wait times include when you're printing a document or waiting on the hard disk or CD-ROM drive. In each case, the CPU is not being utilized. Such cases are candidates for being moved to background threads.