The boilers need well distilled water to turn into steam without leaving scaly residue all over the place. During normal operations the steam is routed through engines where it expands and cools and eventually through condensers where it condenses back into water to be fed to the boilers back again.
If they throttled the engines down, the excess steam would have kept gathering, increasing pressure. They'd need to either lower the coal feed rate - which they did not want - or blow off steam [1]. Ability to blow off steam is a safety measure not meant for regular, extended operations. Think "analogue of nuclear reactor SCRAM". It causes loud release of huge unseemly clouds of steam, but more importantly it causes loss of the valuable distilled water for the boilers.
Btw, operations of marine steam powerplants is surprisingly complex and fascinating subject; consider having a look at this WW2-era navy manual [2] if you ever wonder what it takes to run one such at top power or efficiency.
[0] https://en.wikipedia.org/wiki/Boiler_feedwater
[1] perhaps they could route some of the steam directly from boilers to condensers, but I don't think typical condenser installations are capable of directly taking in HP steam
https://www.youtube.com/watch?v=EgLRSkRioRA
Pretty much everything in Keith Rucker's Vintage Machinery channel is interesting.
Not at all that simple. As already mentioned in the thread, they had limited fuel. Boiling 100s if not 1000s of gallons of water takes more fuel they already don't have.
SWAG: they probably weren't just using distilled water. There was likely some pH additive or other chemical treatment that water uses for a multitude of factors (scaling, corrosion, erosion, foaming, etc etc) that gets lost if you just vent water overboard - lost physically as volatile chemicals are vented or lost logically as new water must be treated.
Also, the boiling process itself is much more complicated than a pot and a fire. To achieve any reasonable amount of efficiency at all requires a multistage process with heat reclamation. If you plan on transiting salt water, the boiling process produces a thick, salty effluent that can be difficult to pump and is strongly corrosive. The weight, complexity, training, parts inventory, etc are all large costs. These are all hard sells for relatively short voyages.
Any ship installation is limited in size, weight, and power, so you carry machines only big enough to produce sufficient supply for normal operations plus a bit of spare capacity. Even if they had a distilling machine(s) on board, they would certainly not be able to supply as much water as they'd be able to boil off in the main boilers.
Also, the apparent ease of distillate production is a bit misleading. In fact, it is a bit of an involved subject, with multi-stage processing, and delicate balances of pressures and the occasional thorough cleaning. At least in the submarine world[1], that is...
Basically you want to heat up the water only as much as you need to, but later on recover as much as possible of the heat from both the distillate and the condensed discharge. This is helped greatly by maintaining proper pressures at different stages. Otherwise you'd be running exceptionally inefficient process, and at scale & over span of weeks things add up to real numbers.
I suppose a visit to archive.org's collection of old steamship books could yield a definitive answer.
I would have thought there would be some valve somewhere that can be opened to vent excess power (steam?) into the atmosphere rather than being forced to drive the propellers faster, is that an incorrect assumption?