I think I grasp the concept, but would someone fill in the details on how and why these engines where used as stationary boilers?

In the case of the power plant, I believe the locomotives were used to provide steam to "blow out" all the piping at the newly constructed power plant before its high-pressure boilers were placed on-line. At Carpenter Steel, the locomotives provided steam for various operations when the steel company's boiler were shut down for some reason.

ok, but how was steam diverted from the locomotive?

A brand new boiler has a good bit of mill scale from the manufacturing process and also there is a fair amount of machine oil from rolling the tubes. The steam piping system will often have welding slag and sometimes even gravel if the piping lay-down area was not paved. I've even seen bolts and bits of grinding wheels show up. If you simply fired up the new boiler/system and put the steam to the turbine all kinds of bad things would happen.

A brand new boiler must be boiled out. One larger boilers this is often done with steam from an outside source due to concerns about maintaining minimum flows through economizers and superheaters as large industrial boilers are not made to run on really low steaming rates. The piping system gets a good blow to make sure nothing is going to be blown into the turbine. These systems need to be hospital clean, a small chunk of metal could destroy a million dollar turbine.

Connecting to the plant is a matter of finding a steam fitting on the boiler. Most boilers will have a 2 inch threaded connection somewhere. This will often do the trick. Locomotives were often called into this type of service in the days of steam. Today there is a whole portable boiler industry that supplies steam for boil outs, blowing out systems, to sustain plant operations while boilers are repaired, peak service in winter, and emergency back-up service.

Back in the mid-1990s; I had the opportunity to tour a power plant that was built as a cogeneration facility (producing steam for the refinery next door and electricity, using non-condensing turbines, one of the first such facilities) starting in the 1930s. It was a an amazing sight to walk down the old firing aisle, with a large brass gauge overhead giving the steam header pressure; and finding the plant had started one of the "newer" (1950s, I believe) boilers with steam driven auxillaries to meet a short term shortage of steam.

Anyway, the older units had tracks that ran into the the turbine hall next to each turbo-generator. I always assumed these were used to bring in the turbo-generators when the plant was being built; but one old hand who used to work there told me they also used to bring a steam locomotive inside to start the old boilers with a charge of steam. I have never heard of this practice before or since.