Successful Web Traffic
For online advertisers, the web traffic data is their way to determine which sites are getting the most hits or most views by their indended audience. Some companies prefer to display advertisements in some parts or sections of their sites, just like in magazines and newspapers. The term for this is Display Advertising.
To make these websites more popular, these pages should be included in the search engines. Two ways to do this is to pay for inclusion or other fees like placement (also known as Search Engine Marketing or SEM) or to simply correct the code, or improve their content to be on top of the Search Engine Results and stay in that position for a long time.
The most obvious reason why SEMs and SEOs are effective is that when people search online, they tend to choose the first few topics amongst the long list of other related topics. Observe yourself to verify this theory.