It's not just the annoyance, slowdowns and bandwidth usage of adverts that is a problem.
Dodgy advertising networks are also a very common way of distributing viruses and other malware. Even reputable sites could have sold advertising space to a network who might have subsold it to someone who is up to no good.
Given the complexity of web browsers nowadays and the number of vulnerabilities that are constantly being discovered and patched, I don't consider browsing without a good Ad blocker to be safe, and my employer force installs uBlock Origin by GPO policy. Browsers such as Chrome and Firefox self-update by their own accord pretty silently, so this arms race might not be that apparent to most users.
I must admit I am shocked when I am forced to turn off adblock to access some sites how many seemingly legitmate sites trigger security alerts from Avira or Chrome.