HTTP is a stateless medium.

Which means a website doesn’t (can’t) remember what you did before on it. Each time you click a link it’s like the first time the website has ever seen you.

This makes it impossible to ‘sign in’ to a website.

The way they get around this is by dumping a small piece of code on your computer that says ‘hi website, it’s me and I have a proper account and am logged in’. That’s a cookie. And yes, websites need them to operate any kind of user experience.

So instead of a page that says ‘who the fuck are you?’ It can now retrieve the info from the cookie and show you the page that says ‘hi, KuromiGirl04, what’s happening? You’re all logged in and can access your account details or carry on from where you were before’.

Originally cookies could only be created, and then read back, by one website. So, eg, if you logged into your account on foobar.com, only foobar.com could read that cookie back.

But someone came up with the brilliant idea of third party cookies. So now, if you visit foobar.com you also (if you agree to it) get cookies created by facebork, grabble, aggressive-advertiser, the nra, the nsa, the kkk, and whoever else has convinced foobar.com that they get some value out of the deal.

That’s where the hundreds of cookies you need to scroll down and deny come from. Mostly advertisers or analytics, or advertisers, web optimisers, or advertisers…. And these third party cookies can be read anywhere by the company that sets them.

That way, when you visit shoefuckeringfreak.com facebork knows you’ve visited it and suddenly starts showing you sexy, sexy shoes on your facebork feed. And so forth.

They store data to later recall on the site. Like if you want to stay logged in on subsequent visits without going through the login process, that needs a cookie.

A cookie is a small piece of data a website can store on your computer, and it will be included with every request your browser makes to that website. While most visible cookie usage (I.e. the endless popups) is just to track you and violate your privacy, their are plenty of legitimate uses for cookies:

• store settings you make on a website, such as dark mode or (ironically) that you don’t want to be tracked
• store a session ID so the website can store more data on their end, such as the contents of a shopping cart. This is also often used for analytics, which is basically following the paths users take through a website to see if everything works as intended.
• store a successful login to a website. Most websites will require a cookie to allow you to log in. There are other ways to accomplish this but they’re ancient and annoying.

Basically you should keep declining cookie popups, except for functional cookies on a website where you actually want to do stuff (login, shop, etc) instead of just reading something.

The basic type is for remembering stuff for when you return such as being logged in

For your layout preferences, colours, other UI options, states (what ‘back’ means), things like that, usually.
If you have an account, what page you were on last, your history so it can give you related items, and other ‘convenience’ features.

Less ethical sites use them to advertise to you, and (more rarely) some scammers use them to scam you.

Most are benign, but some are not.

Cookies are essentially an older way to store data for web pages. The browser knows to attach this information on every request to the website.

Where they got a bad rap is companies like Facebook, Twitter, ect. Started embedding them in the “like” buttons on pages passively tracking every website you went to, whether you were logged in or not. Gaining massive tracking of the whole Internet.

Once people started getting wind of this there was massive pushback against cookies and various legislation was for sites to have to tell you that the site uses cookies for various things and how they track your activities.

Theres already some great answers to your actual question, but id like to add some context to the underlying question.

Firefox now does tab containerisation by default. Cookies are only shared between tabs for the same domain. This defeats the main problem with cookies - tracking you around the web.

Theres an addon called “i still dont care about cookies” which just accepts cookies dialogs, because there’s no good reason not to accept them really.

LibreWolf (or other plugins for firefox) can purge cookies when you exit, but having to log in to everything every time really sucks when a lot of sites are only doing email 2fa now.

More recently, sites track you with fingerprinting which is very difficult to defeat. Sites gather all sorts of information about your device and use it to identify you. LibreWolf is hardened against this but my browser is still “unique” when tested.

as others have said they can be used to help remember stuff like log-in status, but also stuff like what you put in your shopping cart if you leave and return.

Now you also mentioned tracking, and yes, thats why you are asked to accept them now. websites and marketers use cookies to track all kinds of things about who you are, where youve been, and what you do so they can target ads to you. This can be info like where you are located in the world, in your country, even in your state. What OS you use, what browser you use, the size of your screen. And if you log into the same sites on your phone then they get all that info too.

There have been massive lawsuits about the dossier different companies built using these tracker cookies and even today it is very difficult to remain truly anonymous online because everything is tracked, legally or otherwise.

Why the hell would you reject cookies? They’re yummy! 😋

It’s to identify anonymous people online and track their activities. Eventually you may fill something out and become known and they’ll continue tracking you but as a known entity. Often this is used for marketing, but also for your convenience.