전하다 is passing something on to someone who is not present: tell him thank you, tell her I'm sorry, please tell the driver I'm grateful. -다고 wraps up the message being relayed. The passive 전해진다 or 전해집니다 does a different job — 'it is said that' — the narration voice used for legends, history and, in these lines, jokey captions written in the same tone.