Bad spark plugs can cause the engine to stall or be hard to start if the spark plugs are wearing out and not providing the proper gap for the proper spark.
The spark plugs create the spark that combusts the fuel and air and creates the explosion in the engines cylinder to make the engine run.
So if your car is constantly stalling or hard to start and you haven't changed the spark plugs within 100,000 miles or more then it would be a good idea to replace the spark plugs anyway.
Other things that can cause the engine to stall are faulty fuel pump, bad ignition timing, low fuel pressure, clogged fuel filter, bad fuel injectors, clogged fuel injectors etc.
Spark plugs are usually not the cause of a car stalling but bad spark plugs can cause stalling sometimes.
Another cause of a car stalling is due to a clogged catalytic converter.
Spark Plugs should be replaced every 100,000 to 150,000 miles as preventative maintenance anyway but check the fuel filter, fuel pressure and the fuel injectors as well.