Some sites are filled with blinking scams, phishing attempts, fake buttons, attempts at tricking people to install malware, and what have you.
Some to the degree that they are literally unusable without ad-blockers, because actual content is hidden behind spammy scammy phishy infectious ads.
Some to the degree that they are dangerous without ad-blockers.
Some to the degree that they are merely annoying without ad-blockers.
Without such sites, ad-blockers wouldn't be as attractive as they are, and people wouldn't install as many ad-blockers. Once an ad-blocker is installed, people tend to have them enabled on all sites.
In my view, using ad-blockers is a defensive tool in the arms-race against scum-ads on the internet.