This can be used for for instance homework assignments where the teacher opens a topic and posts an assignment, and all students make a post with their answers. When everyone has submitted their work, the teacher can open up the answers so the students will be able to discuss them.
A second use case can be an auction where something is offered for sale. People can post their bid and when the sale is done, the topic owner can reveal all bids in order to show the community that everything went according to the rules.