It can be used to reduce pH in water systems. It would be injected with a chemical injection pump. We also use it in swimming pool capital of the world Phoenix to acid wash our swimming pools, a nasty job.
And as a fluxing (Diluted) agent when soldering Galvanized (Zinc coatings) steel
For descaling copper like a chill water system we try to keep away from acidic chemicals when possible as any residue acid will continue to eat away at the base metal.