I think the best phrase for the overall phenomenon would be 'side benefit', or 'gravy'.
something good that happens in addition to a main benefit or purpose
something advantageous or valuable that is received or obtained as a benefit beyond what is due or expected.
But there is also the 'ripple effect', which typically pertains to people directly
In sociology, it can be observed how social interactions can affect situations not directly related to the initial interaction, and in charitable activities where information can be disseminated and passed from community to community to broaden its impact.
And there is also 'windfall'. Taken literally, it refers to 'something that has been blown down by the wind'. Figuratively, it refers to
A sudden large benefit; especially, a sudden or unexpected large amount of money, as from lottery or sweepstakes winnings or an unexpected inheritance or gift.