It's just two breakers in the traditional space of one breaker. Double pole breakers are typically used where you need to add a new circuit and are out of slots in the box to add a breaker. So you remove one existing single breaker and replace it with a double pole breaker. If the double 20amp breaker you pictured has only one wire to it then it's using only one 20 amp breaker. And you have a spare 20amp breaker that is not being used.
As to why? Could be a plethora of reasons why there is a second unused breaker. It could be that they ran out of single breakers on the assembly so the installer just used a double. Or the original failed while on vacation and the owner could only get a double in the small town they were in.
