The power of steam, Why?

Why is a boiler only legal unattended up 15# of steam. What are the magical property of this life giving substance called Water that we take for granted? That thing that inhabits 65% of who we are. But more importantly the power it gives man in it's abilities.

I was visiting a company who had a lists of needs. One being their steam boiler. I toured the facility comprising of a two story office building in the front with a balcony outside, all glass with offices. Now there were two more large production spaces about 8,000 sf each. The building was a steel structure with a sheet metal sheathing for the shell. We call them butler buildings.
(I don't know why?) To the very rear in a pit was the steam boiler. Now I had picked up talking with the maintenance personal that they were having trouble with it.So I noted that along with the other things.Now the boiler was the size of a travel van, the type hotels and shuttles use. It was a low pressure vessel. So I submitted my quote and heard I lost the bid. Which was fine I didn't invest all that money and time into it. A month later I read in the news that the building exploded. So I said to my self that I wanted to stop and see it. The explosion was so vast and destructive in my mind. Standing from the street it was like a miniature nuclear bomb went off. The shell was blow off the steel completely, the roof gone. shards all about. I walked into the building or what was left. The offices glass was all gone, discentergrated, and the dropped ceiling was laying completely intact on the office floor burying all the desks. On both floors. Strange event I thought. The extrusion press of many tons of steel did well but stripped of all accessories. Everything else was gone....somewhere. The boiler gone. I stood in the center and looked up to the sky and noticed an I beam, an 18" steel I beam bent at a 90 degree angle and twisted 45 degrees. It was then that it hit me as to the destructive power of the boiler when it went off. I just stood in awe at the power of the steams force. That always stayed withe me. As a reminder to the awesome power of steam. Later I heard that no one was there,which was good.
Why does steam posses such a destructive force.
What was the force that hit the face of that beam to bend and twist it ?
What do you think was the total release of energy at the moment of the explosion.
The boiler was 50' away from the beam.