The human eye loves the circle and embraces it.
While I am a strong advocate of laying out posters on a grid, you can end up with a poster that is relentlessly rectangular. A circle can be a strong antidote to a poster filled with right angles.
Circles can be used to draw attention. It is no accident that circles are used in those ubiquitous bullet lists.
Circles can create tension. Like a ball, they suggest something that is mobile and not static.
Circles can be used to create white space. As everyone knows, a round peg will not fit into a square hole without leaving spaces.
Because they do tend to break, rather than reinforce, grids, circles are probably best used in small doses on a poster. A poster without a circle will not be noticeably missing anything. But it’s a useful exercise to consider how you might work a circle into a poster grid.
Elam K. 2004. Grid Systems. Princeton Architectural Press: New York. Amazon
Photo by user Oranguthingy on Flickr, used under a Creative Commons license.