When an electronic unit controls a solenoid, a DC motor or a headlight, the current load in the final circuit may be substantial. A control unit could deliver current directly to the circuit, but that would make it very expensive since such an arrangement would require a pricey solid-state driver to toggle the high amperage. It would also require a large heat sink to dissipate waste heat the driver would generate, which would add weight and complication (along with cost) to the vehicle systems. It would also detract from long-term reliability.