You will find a lot of Gold Coins while you explore the hotel in Luigi’s Mansion 3, but it's not apparent what they are used for to begin with. The thing about the Gold Coins is that you cannot use them for anything until a certain point in the game, and it's only after you find and rescue Professor E. Gadd that you can use them for anything at all.
After you have found Professor E. Gadd, you will be able to teleport to his lab at any time, and it is in there that you can spend your Gold Coins. He has a small shop there where you can buy 3 different items:
- Gold Bones. This item allows you to spawn at the same spot again when you die, so you don't get the Game Over message. This is super useful for boss fights where you are close to beating the boss, and will allow you to have another go at it without the boss resetting back to full health.
- Boo Finders. Used for finding Boos easier.
- Gem Finders. Used to find Gems easier.
Each of these items cost 1,000 Gold Coins each, and are consumed when used, so you will need to save up a lot of Gold Coins if you want to use these items to make your life a little bit easier.