Without them, every time you load the website, it would be as though you’ve never visited that website before.
If the website doesn’t know who you are, then you won’t be able to login. You would not be able to buy anything online, because the website would forget what is in your shopping basket.
However, cookies can also be used to remember your preferences and actions on a specific website, and so they can target adverts to suit. Some websites share cookies, so they can build better pictures of what to advertise and how to target customers.
If you have concerns over what information is being stored in these cookies, you have several options.
You can turn off all cookies, though this will prevent many sites from working at all.
You can turn off 3rd party cookies. This prevents most websites from sharing your information with other websites. Some browsers do this automatically, but you should check your settings if you wish to use this option.