To protect your data

• Servers should show you data that only you should be allowed to see.
• Browsers should give servers data that only they should be allowed to see.

What should servers show you?

They should show you public information, but hopefully, the will show your data to only you.

What should browsers give servers?

Browsers should prevent servers from accessing any data on your device.

But how can the server know it's you?

When you log into a site, you give it your password, and the site verifies it is you. But in order to not have to ask you for your password on every page, it needs to save somewhere "proof" that you're logged in securely.

A cookie file is the best way to do this (More about why we need cookies)