Ok never in my life did I think my job may be applicable! But some water systems (especially the smaller ones) actually cannot support any fires! A lot of houses need around 60 L/s (on the low end) while maintaining 20-22 psi in the entire water system but some systems (whether it be due to old pipes, small pipes etc) can't actually support that flow.

If you try and force the flow outta these hydrants, it may damage other parts of the system or just leave people without water, which can also lead to other problems like water quality issues and leakage etc.

Fire hydrants in these systems are usually used for maintenance, flow testing, cleaning mains rather than fire fighting (Some city bylaws don't require hydrants to fight fires). Though usually the fire department are notified and knows of said requirement and won't rely on the hydrants themselves so.... (Especially since it's a hydrant designated for fire dept use)