Cookies have non-infringing uses, like identifying you to Lemmy’s Web interface so that you can post from your account with the settings you’ve chosen for it. Problem is, even sites where they have a proper purpose don’t set them at the appropriate time (as part of the login process, or when you first add something to your shopping cart for ecommerce sites).

Ad tracking has absolutely no uses that benefit the user, unless they’re the type of weirdo who actually clicks on ads voluntarily, which I’d guess is less than 1% of the population. Those people can use the opt-in toggle if they want.

Not all cookies are harmful and some websites don’t work properly without cookies. Having cookies off by default also usually means user preferences wouldn’t be saved when you leave and return to a website.