I tend to do these 'test' pages when I'm excited about doing a story. I just play with format. The below is an idea in 2006 that never went anywhere.
I don't tend to just write the words, sketch some characters, and off I go to the drawing board. No. I like to gauge the book as a whole in my head. I like imagining how the pages in the book will be. The text. The page borders. The panel/grid structure... basically the 'feel' of the pages being looked at when/if the story becomes real. It helps evoke a mood for the work. Helps convince me, "Yep, this is working."... Helps me visualise, stoke the fires, and move forward knowing that it all makes sense to me.