Poster media come in a wide variety of formats, and can be displayed in a wide variety of locations. Due to their printed character, they offer an infinite number of design options that can be adapted to suit the respective context. You can't just click them away - and you wouldn't want to, either.
Anyone that is out and about in the city sees poster advertisements. That is why a poster campaign quickly reaches a large number of people as they go about their business in a city. It's effective brand advertising.