prototype reversing loops

Most modern reversing loops exist to serve facilities that load or unload unit trains. That way a whole train can be loaded or unloaded in continuous motion. These types of facilities include coal flood loaders, grain elevators, power plants, and rail-to-water transloads.

Throughout history there have been other reasons for reverse loops. They were built when equipment was too large for the turntables already in use and it was more economical to build the loop than rebuild the turntable. They'd also be useful for turning whole consists, either of just locomotives or whole trains, most likely passenger trains. Loops would also be built where heavy snowfall would regularly impede the use of a turntable. I believe the While Pass & Yukon had a couple loops instead of turntables in a few locations.

  In Portland Oregon (My hometown) when Union station was really a Union Station........ It was served by the Southern Pacific,Northern Pacific, Great Northern, Spokane Portland & Seattle, and cough-choke Union Pacific.  After the train had arrived the power was removed and serviced. The Portland Terminal RR would take the trains out to their yard with balloon track (reversing loop). After turning the train was put back into Union Station and serviced. That way the continuity of the train was not broken up, and less switching was involved.

Jacko 

PS: The Portland Terminal RR had some Alco T-6 "notch-nosed" switchers, painted in grey we used to call 'em the grey ghosts.

SP used reversing loops and wyes for turning helpers, here's one in Truckee, CA. near Donner Pass.