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In automotive applications, relays serve an essential role in controlling high-powered devices using low-powered switches, making option B the correct answer. A relay functions as an electrically operated switch that allows a low current from a control circuit (such as a light switch) to activate a higher current circuit (like headlights or electric motors) without the switch needing to handle the heavier load directly. This not only protects the integrity of the lower-power switch from excessive voltage and current but also reduces the complexity and size of the wiring needed.

Using relays enhances safety and reliability in vehicle electrical systems, as they can manage the demands of multiple high-drain components without overloading lighter control switches or circuits. For instance, using a relay to control the headlights means that the switch only needs to carry a small amount of current, allowing it to be designed for lower electrical loads.

The other options do not accurately describe a typical function of a relay. Charging the battery is typically managed by the alternator and battery management system rather than through relays. Measuring electrical current is performed with specialized instruments, not relays. Providing constant power to lights typically does not involve relays, as that would entail a direct connection rather than switching, which is what relays are designed to do.