It is not a good idea to force an ad for beer on an ex-alcoholic/someone trying to stop alcohol, for example.
Things such as gambling, smoking and (though somewhat debate-ably) alcohol all fall into this category.
Regardless of whether you agree with it or not, many countries now ban the advertising of cigarettes, so realistically, it's not too alien of an idea.
Allowing users some granularity over ads makes sense for everyone. I am never going to buy nor rent a car so showing me car ads is stupid. Allow me to opt out of seeing car ads and the advertisers save money and the content I'm watchig doesn't get negative associations and I am less likely to install ad blocking software.
But if I create a video and host it on YouTube I would want to prevent adverts for gambling or alcohol to appear next to that video. I'm not sure this is offerred by Google.
I find YouTube ads to be just weird and annoying.
i) I get gambling ads. This is a sleazy destructive industry and people should have to opt-in to receive ads for gambling products. (Ditto alcohol).
ii) I will queue up some obviously children's content for my son but get an adult's ad (eg ads for violent film) before that. This is fuckig stupid and it's disappointing that Google gets this wrong.
iii) I'll go to watch a 30 minute video but get a 5 minute ad. (Or maybe 3 minutes?) I should start making a list of the company names that do it.
iv) a lot of the ads are just terrible
Google really needs to implement some controls so people can avoid alcohol and gambling ads.