They're supposed to be useful, ie the site should not be stalking you until you say yes, the consent prompt should not be misleading (no dark patterns) and service shouldn't be denied until you say yes (I'd argue taking over the entire page and preventing you from browsing until you make a choice would be against the regulation).

Note that this is not just about cookies but about any kind of stalking; browser fingerprinting, local storage, etc. It's more of a legal thing where you gave them permission to track you as opposed to a technical restriction, so a browser-based solution isn't really an equivalent (you can block their cookies but they'll still fingerprint your browser).

The problem is, a lot of them are badly implemented and load the trackers regardless, a lot are badly configured and consider Google/Shitbook as "necessary" cookies you can't opt-out of, and most of them are a pain to opt-out (with Yahoo/Oath - the cancer of the internet - in prime position - their GDPR flow is outright offensive).

Sadly until there's actual enforcement of the regulations things will stay that way.