Cookies are used to remember each user's preferences including their location. Once a cookie is installed in a user's computer, every time the user visits that particular web page their information will be automatically retrieved by the website. Users can choose not to allow cookies.
The reason cookies are used is to enhance the user's experience to bring them specialized pages in line with their set preferences. This means that someone who orders a product on a site can save their card details and the next time they are on the site ordering something the details will be pulled up for them, thus making it easier and adapted to individual users. Cookies are usually cleared by users after a certain periods of time. This is why it is not recommended to run tests that run for longer than one month, as it may introduce the sample pollution.