If you trade something and get gold you can use the gold to buy things. If you keep your gold and give someone a piece of paper that’s proves you are giving them your gold, you have doubled your wealth. You still have your gold but the receipt is just like spending the gold. It’s like prostitution. You have it. You sell it and you still have it.
Banks operate sort of like pimps. They sell something they don’t own, (your gold), over and over again by using paper that’s sort of a promise the paper is worth the same as gold.