What is the rationale of having a "closed" congressional hearing (viz: Comey testifying Dec 7 2018) AND asserting that his testimony (via transcript) will be released 24 hours later?
A standard answer might be that a closed door allows Comey to speak freely, and the intervening 24 hour period allows for classified information (or otherwise sensitive information) to be redacted from the transcript prior to publication. However, the main reason is it's a political compromise.
Comey has requested public hearings. Many Congressmen want private hearings.
Why does Comey want a public hearing? Here's the letter explain why A couple of reasons seem like they could apply. It mentions Comey's lack of a security clearance. It could be he's still annoyed at being fired and wants to go public. He explicitly is asking Congress to get him permission to disclose FBI materials. There's also a worry that members of the committee could selectively leak information that mischaracterizes what Comey said.
Why do the Congressmen want a closed-door meeting? Possibly to allow classified information. Possibly for other political reasons or for the flip side of any of the reasons Comey would want a public hearing. I'm not really going to speculate there.