Post Only can only be used on a limit order.
This condition specifies that if your order can be matched with anything on entry for an immediate execution, you want the order cancelled.
The risk of creating an order for "Post Only" is that you aren’t guaranteed an execution. The market could move and you might never get a fill.
Who would want to use this condition? In an order matching marketplace, there are takers - who take liquidity away from the marketplace - and there are makers, who add liquidity to the market.
Makers are rewarded for helping to make the market more efficient with a lower fee.