Steam direct from a boiler contains microscopic droplets of liquid water. This steam must be superheated to vaporize these droplets. If this is not done the droplets will pit the turbine blades and can cause premature turbine failure.

Before the development of zirconium alloy fuel pellet cladding for nuclear reactors the reactor itself could not be operated hot enough to directly superheat its steam. So early designs proposed "hybrid" reactors, using a nuclear reactor to boil the water and make steam and a fossil fuel plant to superheat the steam. But zirconium alloys were developed before any "hybrid" reactors were actually built.

Most steam engines except those used on tourist railroads have been replaced with the more efficient steam turbine engines.

The orgin of blow off some steam comes from old steam boilers. To let "off steam" or "blow off steam" would be to relieve the pressure in the boiler, to avoid risk of damage. When talking about a person, it means they are venting their thoughts, as a form of stress relief.
To calm down and or from getting mad.

Throughout a hundred years or more steam power was constantly improved by new devices. The regulator, safety valves, compound cylinders, different pistons,etc.

From the start of time, about 13.8 billion years.