Exercising the thermal mass of a boiler will improve system performance, says John Siegenthaler in his Hydronics Workshop.
All physical systems, whether mechanical, electrical or thermal, need some type of stabilizing effect to prevent rapid fluctuations in their output.
A flywheel provides this stabilizing effect for an engine. The rotational inertia it adds prevents the engine from abruptly jerking to a halt when there's a sudden but brief increase in its loading.