Many ads are malicious. Many try to install software you didn't ask for on your computer, or they try to trick you into pressing a "download" button on a download site that isn't for the thing you wanted but for their software, or they fake error messages to trick you into installing malicious software.
Back when I was younger, all those sort of activities would fall under the banner of "cyber-terrorism". Therefore, installing an ad blocker is an act of counter-terrorism.