The goal is to attract attention. They even generate excitement. Color is the key. Use a big banner's color to contrast with the businesses usual color schemes. Use colors to show change as well as having the banner announce change in its lettering.
Big banners are promotional.
As a new addition to the visual landscape, they are by their very nature a demonstration of something new. The bigger the new banner the bigger the sense of promotion. Bigger is louder. Signs in the window are restricted to the size of the frame. Banners have no such restriction.