It's called Advertising, or publicity. The more places you advertise the more people you reach, so to maximise to possibilities, you need big ones, little ones, ones that stick on buses, in windows, bill-boards, shops, bus-stops, anywhere people may see it and want to go to a view what's happening..
Don't forget, when posters started, there was no Internet, T.V. or other means of advertising your promotion other than posters, or word of mouth. As the digital age moves on, we are seeing a lot less paper advertising, as they are now not need as such.