The need for heater (or steam) cars only came about after dieselization - passenger cars were almost always steam heated at this time, and nearly all steam engines had provisions to pipe steam back to those cars for their heat. Freight diesel-electrics, the majority of locomotives, weren't built with the necessary steam generators, so railroads would build cars like GN 8 to supply heat to trains whose passenger locomotive had broken down. If I'm not mistaken, they used them for supplemental heat in the Upper Midwest and Canadian winters too, even when the trains already had steam-equipped locomotives. Couldn't afford a malfunction of one of the heaters when it was -40. Ever since the 1970s though, cars have been built to use electrical head-end power for heat and not steam, so the steam cars aren't needed even by Amtrak. They were known to use similar cars supplying HEP for a while though.
Here's an example of a later one built from an F3B, GN 16: http://www.mtmuseum.org/jsr/roster/gn16.php