They didnt melt them to make into jewellery, but they turn it to the bullion to make into metal bars, and what are they going to do with the metal bars is out of my mind, I cant never tell why they want to melt it at the first place.
An example might be copper coins. In many countries you still have them and to be honest - usually, their value as of the currency is lower than of metal itself. And copper can be used in a variety of industries - from cables to cooling systems. Same might go with other metals when needed.
Of course, this would apply mainly to relatively modern coins, as old ones have historical value far exceeding the value of the metal itself.