"An old, commemorative coin. It has something that looks like mold sticking to it."
The Dirty Coin is only meant to be collected to be later transferred to the player's save file in Resident Evil 7 Biohazard.
Resident Evil 7 Biohazard
If the player obtained the Dirty Coin in the demo, it will appear in the Item Box of the main game. The Dirty Coin functions as an extra Antique Coin, which is used to unlock power-ups or weapons through the course of the game.
The Dirty Coin is located in locked attic room on a signboard. It is made accessible by solving the Bloody message puzzle.