Answer:
Advertisers mainly use demographic data to know better their target audience and to be able to segment it. Having data on gender, age, income, (the three main demographics), languages spoken, parental status, type of family, etc, allows advertisers to divide the market into smaller groups with common characteritics called segments.
By using segmentation, advertisers can elaborate a tailored marketing strategy that more accurately targets the audience of each of these segments, ensuring that the $50,000 destined to adversing is properly spent.
(disclaimer: this isnt mine)