The boiler isn't the only source of heat in many hydronic systems.
Too bad Btus are so dumb. Instead of complying with our simple concept of following the arrows, they have this “thing” about temperature - always wanting to move from warmer to cooler. They don't care about what we want them to do. They don't care about the arrows on the schematic. They don't even care if the building is properly heated as they move about.