An advertiser would benefit from using affinity audience targeting if they want to reach people:
- regardless of their particular interests
- with a very specific interest, for example, avid marathon runners
- who’ve already visited their website
- with a particular broad interest, for example, sports fans
Adding audiences allows you to reach people based on their specific interests as they browse pages, apps, channels, videos, and content across YouTube and the Google Display Network as well as on YouTube search results. You can select from a wide range of categories — from autos and sports to travel and fashion — and we’ll show ads to people who are likely to be interested. Depending on your advertising goals and the stage of the purchase process your customers are in, you can choose to add different audiences to your ad groups.
Select from these audiences to reach potential customers at scale and make them aware of your business. Don’t confuse with affinity and custom affinity audience. Both are different. That’s why the correct answer is “with a particular broad interest, for example, sports fans”
With custom affinity audiences, advertisers can create audiences that are more tailored to their brands, compared to our broad, TV-like affinity audiences. For example, rather than reaching Sports Fans, a running shoe company may want to reach Avid Marathon Runners instead.
Read more here: https://support.google.com/adwords/answer/2497941?hl=en