Another reason is to collect data on the webpages you visit for data mining and improving web search. The more data a search company has, the better it can rank

Another reason is to collect data on the webpages you visit for data mining and improving web search. The more data a search company has, the better it can rank results and personalize ads. See these papers from Microsoft and Yahoo: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.161.3605&rep=rep1&type=pdf http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.161... http://portal.acm.org/citation.cfm?id=1557019.1557135 http://portal.acm.org/citation.cfm?id=1557019.1557135 http://portal.acm.org/citation.cfm?doid=1835449.1835548 http://portal.acm.org/citation.cfm?doid=1835449.1835548