Planning a multimedia photo essay or audio slideshow needs a sort of bird's eye view of the project as a whole, and that's what storyboards are essentially supposed to do. Storyboards help to pre-visualizing the photo essay's sequencing, and act as blueprints for the project.
The above photo shows my rather elementary storyboard for one my recent Bali audio slideshow Ngaben: Cremation Ceremony, which has the sequencing of images and audio clips, timing, etc.
Naturally, it would have been more professional/efficient to use a proper storyboard template downloaded from the web, but where's the fun in that!?