The next time I will ask them as I am inquisitive as well, but i can only assume that they do it to make sure these valves don't get stuck in one position, deep below in ground all sort of things can cause these valves to freeze.
When they were doing it outside my house, I asked, and was told there are two mains under the road, and they periodically run the water through one, then periodically through the other.
They run the fire hydrant to flush out the rust and sediment from the pipe that has been out of use.
I suppose that having two pipes makes it easy to use one when the other is being worked on or bursts.
recently they also carried out some work in my street and dropped a card stating that our water supply would be shut off for a few minutes on a certain day and advised us to run water for a few minutes before drinking it.
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