When you return to a website, the browser sends this file to the website's server. This allows the site to recognise who you are and perform certain tasks like: keeping you logged-in to a secure area of the site or personalising webpages to you. They also allow websites to more accurately track your browsing behaviour for statistical analysis.
Due to their important role of enhancing/enabling usability or site processes, disabling cookies may prevent users from using certain websites. Should you wish to disable cookies, however, you can find out how at: www.allaboutcookies.org/manage-cookies.