You need to know because how you market to each group is very important. You don’t market to teens the same way you market to professional business people. Women receive marketing strategies different than men. You don’t want to waste your time and money on marketing strategies that have no impact on your target audience.
Writing a romance book? For women, men, or teens? You should not market a romance book for teens in the style of an adult romance novel. Market to men in a way only women would appreciate will turn them off. Who are you marketing to?
When major companies want to market their product, they know their audiences. If their audience is kids, they market in a way that kids will see it and get it. Ads will be on during kids’ shows and placed where kids will be looking in stores. You will not see an ad for a child’s toy during an action movie. You’ll see ads for beer, travel, cars, and clothes. The right marketing for the right audience.