Thumbnail pages can be looked at as a blueprint or a map to where the finished comic page will end up. They are used as a way to work out the overall design of the page. The page design is a bit more complex then it may seem. For one, it has to convey an idea - tell a story. I view it as how a writer would write a paragraph. An introductory sentence followed by the main idea. Finally the concluding sentence leads you to the next paragraph. Visually, this starts as an establishing panel, followed by panels depicting the action of the page. The last panel is meant to lead you to the next page:
Sometimes the designs change a little by the time the penciled are finished, but the idea remains:
The overall design has to move the reader's eye through the page. This is done with the "Z" principle:
From Will Eisner's Comics & Sequential Art
Without some kind of design, a story can not be properly told. Thumbnails help work out that design.