Piper Aircraft introduced the PA-23 Apache in the mid-1950s as an executive transport and multi-engine trainer.  The original Apaches had 150 hp or later 160 hp Lycoming O-320 engines that provided very limited single-engine performance.  In the multi-engine training environment of the 1960s and 1970s, engines were actually stopped for training and horsepower of the running was often not adequate to maintain altitude.  If the instructor was unable to restart the "failed" engine, safe return to the airport could become a problem.

Current FAA rules allow throttling back an engine to "zero-power", where the thrust and drag from the "failed" engine are just balanced, so both engines are actually running throughout training.  Once a student pilot is ready for a check ride, training is provided in engine shutdown and restart procedures by actually stopping an engine (usually the left engine).  Whether the engine is really failed or simulated engine-out, each Geronimo 180 hp engine provides enough thrust to maintain the minimum training altitude and even climb on the hottest Georgia days. 

The Piper Aztec is a later, six-cylinder Lycoming O-540 version of the PA-23, and can also serve very well in the multi-engine training environment.  However, the more expensive six-cylinder engines have higher maintenance costs and burn more fuel than the O-360s of the Geronimo.  So either airplane will do the training job well, but the Geronimo costs less.  The original, lower-powered Apache is rarely used for training anymore due to the weaker single-engine performance.

The Piper Seminole is a 1970s twin trainer that also has O-360 engines, but has lower single-engine climb performance than the Geronimo.  This is due to the wing design of the PA-23, which generates more lift but is a little slower.  Since training isn't performed to go somewhere, a few knots less is irrelevant.