Flywheel is an important part of a steam engine as it:

helps the engine in keeping the speed uniform.A flywheel is a rotating mechanical device that is used to store rotational energy. In the Industrial Revolution, James Watt contributed to the development of the flywheel in the steam engine. Flywheels have a significant moment of inertia & so resist changes in rotational speed, enabling the engine to run at a constant, uniform motion.