Client wants .story (unpublished files) - do they need this?

I disagree with those who say hand over the source files. The question is whether the contract that you signed with the client is for a service or a product. In my opinion, the .story files are  similar to an "instrument of service". They are the subdeliverable of a necessary step along the way. As an example, what would preventive the client from also asking for the font files that you used, the sketchbook in which you formulate your ideas, or all of the images that you used to create the presentation. The client has not purchased the source files, they have purchased your services.

Your competitive advantage is your ability to efficiently and effectively build presentations from the resources you have built and acquired. Don't give your core competency away.  If the client demands the source files, they should pay a fee for your surrender of copyright.,