It is to prevent short cycling, my boss can get carried away with these things, I could see a 399 firing into a bathroom zone or something but this is maybe overkill lol. The boiler heats the 30 gallons of water and the system cycles on that for a while.
It usually works great with non modulating boilers that are zoned off. It is supposed to have a well style aquastat that the boiler just maintains a set temp with. The secondary side runs on and off until the temp drops far enuf for the boiler to fire again.