I think the picture below speaks for itself. Metal detecting on the beach cleans the beach of unwanted trash and metal objects. Many people would be surprised to see how much trash is left on the beach after a busy weekend. When trash gets dropped on the beach it gets covered by sand almost immediately . Then when a storm comes through it goes out into the ocean. This can poison and destroy the environment. Beer/soda cans and caps begin to rust and can cut people and injure wildlife. Fish hooks are all over the place. There is nothing good about trash on the beach.
While metal detectorists do find gold rings, silver rings, and coins a majority of the things they find are trash, as shown in the picture. In fact, almost everything metal detectorists find is trash. Ask a pro what their trash to gold ratio is and its probably very high. The trash shown in this picture was found over the course of a few hours of searching on the beach. On the far left you can see a big chunk of sharp metal that was only inches in the sand - something someone would of stepped on and cut themselves on.
So if you want to keep your ocean and beaches clean, please support metal detecting on your local beaches and the next time you see a detectorist out, please thank them for cleaning the beach.