I assume city ordinances where I live make it illegal to place permanent signs on busy street corners. That’s probably why, on most weekends, you’ll find people holding signs on those corners.
It seems to be a relatively new phenomenon born at the intersection of the need for short-term, inexpensive advertising and rising unemployment rates. I first saw people holding signs a few years ago, when an office-supply store was closing. Today, signs typically display ads for furniture stores, grand openings and grand closings and $5 pizzas.