No mains with CDU unit

Hi, I’ve just received my CDU from Sismo in Spain and wondering why there appaears to be a mains socket in the rear of the unit? Thanks, Howard.

Thank you for your question
It is a backup power interface.
No separate power supply is required for stand-alone use.
When insufficient USB power supply occurs, it can use 9-24V power supply to power the CDU.

Ah, thanks for the explanation, appreciated.