If anything, every time I have to click through one of those banners I wish that activist organisations/politicians had to pay me for having to go through it.
> In most cases, the add-on just blocks or hides cookie related pop-ups. When it's needed for the website to work properly, it will automatically accept the cookie policy for you (sometimes it will accept all and sometimes only necessary cookie categories, depending on what's easier to do).
When I click "I agree" I am not agreeing to cookies but rather to tracking. If the website wants to track me with (say) browser fingerprinting, deleting the cookies will not stop them from tracking me across sessions. Even worse: since I agreed they no longer need to show me a warning, so I may not even notice that I may want to revoke that consent.
If I don't want to be tracked, saying "I do not consent" is the only legally-actionable way. Deleting cookies works now because cookies are still cost-effective, but this won't remain true forever.