Allowing a service to recognize Your hardware, so You won’t have to provide the same information several times in order to perform the same task;
Recognise the username and password You have already provided so that You do not have to resubmit them on each Web page that would request it;
Allow You to share the content of Sites with other people on social networks or let these other people know what You are looking at or what You think (e.g. the “Like” button on Facebook).
You can also configure Your browser to consistently deny cookies (including cookies used for audience measurement purposes). However, in this case, some features of the sites may not work properly.