Let’s say you go to example.com and they’re using a Facebook Pixel for analytics - so example.com can continue marketing that widget to you after you’ve left their site.

Well, when you visit example.com/shop/widget1 you essentially load a 1x1 pixel image from the url ads.facebook.com/pixel?metadata=goes-here and you web browser sends your Facebook cookie along side the request (as it does for every request to .facebook.com).

So Facebook knows what site you’ve visited and what your interest are so that example.com can better market to you, their prospective customer.

The web relies on cookies so you can remain logged in, have a shopping cart, and do most everything else that is stateful (though sometimes this stuff gets implemented in the browser’s webdata cache called localStorage). The container effectively makes the “session” - all data that is identifying - unique to the tab. So you couldn’t open a second tab and remain logged in to your bank account, for example.

the urls are examples; I’m on my phone, not on my laptop.