Why do some multi-zone heating systems have multiple circulators?
Systems engineering tradeoffs.
In general, one pump per zone will cost more, but can (in theory) be matched more perfectly to the zone (but there are only a few typical sizes of pump, so that may not be all that perfect, after all.)
One pump to feed them all will be the right sized pump as some set of zones, and more or less than ideal with other sets open or closed - however, there's a functional range, and if properly designed either method can be operated in safe ranges.
In general, most heating system designers are not too concerned with how much power the pumps use, (they don't pay for it, and it's not generally obvious to the end-user) which can be another downside (for the end-user) of a particularly inappropriate choice of pump(s) for the job. Designers are concerned with not running the pumps into cavitation which can destroy them, and the fluid velocity in the pipes. In some cases more pumps may mean less wasted power, but without working that out the greater initial expense of more pumps tends to be what's controlling.