For the purpose of transparency, when does Scrum say a valuable and useful Increment mustbe...

The Scrum Guide states that "The Increment is the sum of all the Product Backlog items completed during a Sprint and the value of the increments of all previous Sprints. At the end of a Sprint, the new Increment must be ‘Done,’ which means it must be in useable condition and meet the Scrum Team’s definition of ‘Done.’"1 This means that a valuable and useful Increment must be available at the end of every Sprint, regardless of the release frequency or the Product Owner’s request. This ensures transparency, feedback, and continuous improvement.

References:

1: The Scrum Guide2, page 14

2: The Scrum Guide