The injector was an important milestone in the development of the steam locomotive, but it had one drawback; it used up live steam, i.e. high-pressure steam from the boiler that could be used in the cylinders instead. At speed, one needs all the live steam one can get; on the other hand, exhaust steam from the cylinders is plentiful, in fact often over plentiful for its normal task of draughting the fire by ejection through the blastpipe. It was only natural for a variant of the injector to be developed that could use exhaust steam.
The exhaust steam injector was much larger and more complicated than the simple live steam injector, and therefore most if not all locomotives that used one also had a regular injector on the other side. Adding to the complexity was the desire to make it also work on live steam; exhaust steam is not always available, for example when the locomotive is stopped.
Because exhaust steam is at a much lower pressure than live steam, an exhaust steam injector is huge.
They were often known as the poor man's feedwater heater, because they heated up the water they delivered quite a bit. Exhaust steam injectors were widely used in Britain (which almost completely rejected the feedwater heater and on American locomotives that were not equipped with a feedwater heater for whatever reason.