Earth attracts objects creating pressure on everything on its surface. When something takes in latent heat it pushes against enormous atompheric pressure to expand this expansion or combustion in case of fuel later when condenses creates a vacuum equal to tons of atmospheric pressure which can be effectively used.

Simple calculation:
ideal at 100 degrees celcius (373K)
latent heat water=2.257mj/kg
heat produced by coal =24mj/kg
it can produce hence about 18 metre cube of vapor
or lift the atmosphere for 1m in a cylinder with piston dia of 4.8 m
which is of 18 m2 in area hence about 180 ton/m
we are not using this power why? hmmmm....
1 kg of coal can lift 180 ton for 1m ok.......hmm.......