Those who originally choose to advertise in search results and who were getting a ROI (return on investment) will decide at one point that they need to identify other advertising opportunities. With thousands of websites which have the capacity to display their adverts the advertisers can gain further exposure very quickly.
Another reason why advertisers choose to advertise in Google distributors websites is that it gains them further exposure. 60% of internet users do not use Google, so the advertiser can appeal to a wider audience through choosing to opt for distribution channels. Many website users may be looking to buy a product such as a phone, yet instead of coming across a website which sells such a product, they come across an article. If the article is on a website which contains Adsense then inevitably advertisers can use this channel to penetrate their audience.
Another reason why advertisers choose adsense is because they trust Google. The company is renowned for being an ethical company who are fun to work whilst providing free services to millions worldwide. Advertisers feel that money invested with Google is safe. Despite the evolution of click-fraud and its inevitable disadvantages for advertisers they appear to understand that this is an issue which Google wants to stop and hopefully will eventually. Advertisers are happy that Google admits a problem exits and provides refunds accordingly.
The trust in Google also stems from a trust in pricing. The pricing is set by market forces and therefore advertisers never feel that publishers or Google are overpricing the service. This means that as long as advertisers are able to advertise they will continue to do so, if not at the same rates.
Another strong advantage for advertisers is that they can appear where publishers promote their service. An example of this can seen if you consider a publisher who is discussing the benefits of new IT software. If a software retailer appears on the website then inevitably they will be the likely source from which the web surfer will purchase the product. If the surfer is not interested then you could argue that they would not click on the advert.
The service which Google provides has created an opportunity for businesses of all sizes to advertise. Although the issue of click fraud still plagues the service it is till widely regarded as the best. New businesses attempt to promote themselves on the web, whilst established brands alike choose to attract interest in their service using the same technique.