The Coin Bag, when opened, provides a special bonus. For 30 minutes (or more during bonuses), you will have the ability to find the Coin Hunter Pokémon Gimmighoul and catch it. It spawns on average of one Gimmighoul every 1.5 minutes.
When the time runs out, Gimmighoul will stop appearing in the wild. However, you can re-open the bag after waiting for 1 day. After that time, send another Postcard to Scarlet & Violet and you'll be able to open the bag once more.
This is the only way to get Gimmighoul's Roaming Form between both the main series games and Pokémon GO.
