It's like taking a sample of a swimming pool, to check the chlorine levels.
Also just to be clear, clicks are not likely to be randomly distributed between legitimate and spam sites. If advertisers are paying for plays, they need to be able to see that information as well.
If I was spending money on video ads now I would be somewhere between cancelling my ad spend and calling a lawyer.
But you would have nothing to retort if they tell you you're an outlier, or your specific niche has bad actors that skew the results and they'll look into banning one or two of them (and come back to us again when they get replaced, we'll again clear a few token users)
And maybe the edge of the pool closest to the person taking the sample is the deep end where mostly adults swim, but if they walked a bit further and took the sample from the shallow side with the babies and toddlers they'd find out the water is actually 50% piss with more than trace amounts of feces mixed in as well.
If you care about excrement content, that should be another study.
Bottom line: Representative sample https://en.m.wikipedia.org/wiki/Sampling_(statistics)