There are many ways how cookies can be used by websites. Once a user gives its consent for a website to release cookies, they start tracking personal data, user’s browsing activity and even IP address, purchase history and preferences. And with that information websites can do a number of things:

1. Optimize website performance. Website owners want to have information on how the website is performing, how users spend time on it, and for what reasons they decide to exit. Cookies track user actions and help website owners to improve the browsing experience.
2. Run website analytics. Websites use cookies to also gain insight into how their users behave. They track geographical location and preferences in order to optimize their website for particular regions they are targeting so they can drive more traffic.
3. Advertise. Websites will also collect data with cookies so they can show the most relevant ads. This practice can boost revenue and sales as visitors are able to see ads suited just for them.

Another way how websites use cookies is to perform certain functions. Users cannot opt-out of such cookies if they want to access all the features of a website and they are called strictly necessary cookies. These cookies are required, for example, to save user information such as login details or to keep user items in a shopping bag even after the browser's window is closed. Without cookies, websites wouldn't be able to remember such things. Therefore, they are the most important cookies for a website to function properly.