The first metal detectors were meant to help miners. They were big. They cost a lot of money. They used a lot of power. And worst of all, they didn't work well. People kept trying to make them better.
Metal detectors got smaller. Now they are light and cheap. They also work better. That is why people bring them to the beach. They can look for rings in the water. They can look for phones in the sand. Metal detectors help them find these things. They usually just find junk though.