They are necessary for the website to function and cannot be switched off. They are usually only set in response to actions made by you which amount to a request for services (setting your privacy preferences, logging in, filling in forms, etc.). You can set your browser to block or alert you about these cookies, but some parts of the site will not then work.
They allow us to count visits and traffic sources, to measure and improve the performance of our site. They show us which pages are the most and least popular and how visitors move around the site. If you do not allow these cookies we will not know when you have visited our site, and will not be able to monitor its performance.