Why do you think atheists put up billboards and bumper stickers?

Just wondering what people think the motive is of these few atheists who put up billboards, have bumper stickers, etc., declaring not believing is ok.

Do you think they do it because they believe atheism is for the greater good? Would help society? Or, maybe because they feel it helps the individual? Or, to fight back against all the Christian messages around us; try to demonstrate to Christians how it feels to see their message everywhere?

I’m curious to know if you are an atheist or theist. Also curious whether you think it is a good idea for atheists to do this sort of public display.