Let me give an example. I teach public speaking and I often use online-stopwatch.com as a timer for quick practice speech stuff in class. I had not installed an adblock on the computer in the classroom because I was trying to be good and give that very useful site the ad views.

Then one day in the middle of class I start hearing sound coming out of the speakers which disrupted the class. The site had put an ad on it with an autoplay video. I put ublock origin on it right away.