If your car won’t start it tells you if it’s your battery or something else. Your battery could have good voltage 12v, but when you turn the key it falls off a cliff. It’s hard to tell why. By measuring what voltage the battery drops to while turning the key, tells you if it’s the car battery or one of dozens of other causes.

Happens a lot when it’s cold out because batteries don’t like the cold.

Think about how much you would impress someone knowing this when their car doesn’t start and you can’t even drive 🤣. They would think you are a genius.