One of the 39 categories of forbidden labor on Shabbos is "extinguishing" (mechabeh); how exactly was it used in a constructive fashion when building the Mishkan?
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Rashi (Shabbos 73a, ד"ה מכבה) says that it was done when boiling the herbs to make the various dyes. (If the fire was burning too hot, they'd have to partially extinguish it so the dye wouldn't be ruined.)
However, Tosafos (Shabbos 94a, ד"ה ר' שמעון) argue that the artisans would have been careful to make the fire the right size to begin with. They say instead, as Shalom wrote, that extinguishing a fire was done in order to produce charcoal for metalworking.
My understanding -- and someone please correct me if I'm getting this wrong -- is that for the metalworking they needed to do, an ordinary wood fire didn't burn hot enough.
The alternative was something known to ancient societies, that if you take a pile of wood, cover it with clay so no oxygen can get in, and then burn it, instead of "burning" (oxidation) into ash, it pyrolizes into charcoal. You then put this fire out, and light up your charcoal for metal-working; it burns much hotter than a wood fire, and enables a lot more things to be done with metal. So that initial "extinguishing" was constructive towards an end-goal, not just "eh we don't need the fire anymore."