An unused credit card is one you have activated, but not used for purchases. While not using a card doesn't hurt your score, you can take a credit hit when the card provider cancels the account due to inactivity.
Payment History Benefits
In some ways, you are harmed by an unused card in that you miss a chance to boost your payment history. Payment history accounts for 35 percent of your FICO score. To build one, you have to make purchases and then make on-time payments. Even if you only make one small purchase each month and then pay the balance quickly, you establish a favorable credit history. The longer the account is active, the longer your credit history is as well, which is favorable to your score.
Card Closure Concerns
Banks don't make money when you don't use a card. After awhile, providers cancel inactive cards to prevent further investments in maintenance and communication. Card cancellation reduces your debt utilization rate, which Bankrate says affects 30 percent of your FICO score. The greater the available limit on the card, the more significant the impact losing it has on your utilization ratio.