Websites produce traffic rankings and statistics based on those people who access the sites while using their toolbars and other means of online measurements. The difficulty with this is that it does not look at the complete traffic picture for a site. Large sites usually hire the services of companies such as the Nielsen NetRatings or Quantcast, but their reports are available only by subscription.
Many small businesses credit SEM for attracting new customers. Depending on the competition in your business, relying on unpaid, organic Web traffic via SEO may leave your business far back in the rankings. By paying for SEM, you have more control over the specific keywords that will promote you, as well as where you will appear on the search results page.
Not all web traffic is welcomed. Some companies offer advertising schemes that, in return for increased web traffic (visitors), pay for screen space on the site. There is also "fake traffic", which is bot traffic generated by a third party. This type of traffic can damage a website's reputation, its visibility on Google, and overall domain authority.