Poster advertising is highly prominent and does not require the consumer to do anything to access it. You don't have to tune in or click onto it or turn a page. At the same time, most people regard it as less intrusive than other methods of advertising. In locations where it sits in front of a captive audience — on public transport or at waiting places, for instance — it can even be seen as a welcome distraction.
Often, the paper bus ticket is used as an advertising space. The ticket rolls for the ticket machines are pre-printed on the rear with a particular company's advert.