How does a water mains booster pump work? Isn't the flow rate solely determined by what reaches the property in the first place?
The assumption is that there is a restriction in the pipe and higher pressure differential across that restriction will lead to more flow.
If the restriction is downstream from the pump (a semi clogged valve for example) the operation is obvious.
If the restriction is upstream then the pump will draw a partial vacuum between it and the restriction which will increase pressure difference across the restriction up to some limit.