I don't do it consistently but am always happy that I took the time to do a little thumbnail study before starting a sketch. In this case, I considered both landscape (horizontal) and portrait (vertical) formats. In doing so, I also planned what would be in the frame and the composition. Not only did it help me decide on the format, but I also decided to eliminate the tree behind the building to emphasize the interesting roof shape.
Many people use these thumbnails as a value study also. I was using watercolour crayons and having trouble with controlling their values, so went with the flatter, graphic approach.
Often I do the thumbnail and then redefine the frame. Many times his has saved me from running out of paper, simply by taking a minute or two to plan before starting.