When you pay down your debt by returning USDJ to your CDP. You will be charged outstanding fee proportional to the amount of USDJ being returned. The fee is payable in JST tokens. In this context, the JST is required to use a feature of the system, and thus behaves as a utility token.
It's important to ensure you have enough JST in your wallet to cover the fees owing on the amount you are returning or the transaction will fail.