There's usually a certain audience that you want your advertising to reach. The demographics setting is where you can choose things like preferred gender, age range, education, nationality and other settings that relate to the population.
Each WADS has different options available. Search engines have just a few settings. Social media sites have a lot more. Facebook and dating sites have the most, because their customers fill out extensive profiles.
Go back to your pre campaign research. Reread the affiliate program's offer page, the merchant's sales letter and the persona sheet you created. Who is likely to buy what you're advertising? Is it senior women with a university degree, young females between 18-25, married men over 40?
Use these settings to your advantage, because you probably don't want to show ads for a "reverse mortgage" to anyone under the age of 50. Or show ads for "super combat game" installs to grandma.
At the very least, I usually omit anyone under 18. Yes, they have a certain amount of influence, but they don't have money, or make household buying decisions, and I really don't want to sell anything to minors.
Another factor is the WADS itself. Dig around their website to find their demographics. Bing has mostly adults and seniors, with substantially more women than men, using the service.
That's why I like Bing the best. Women spend something like 90% of household income and make 70% of the purchase decisions. Plus, as people get older, they tend to have more disposable income.
Knowing Bing's demographic helped me know what types of offers work best on it. And make a lot of money by advertising there.
If you're trying to persuade people to do something, or buy something, it seems to me you should use their language, the language in which they think.