Your client who sells sports shoes might choose to target a custom affinity audience if he wants to:
reach men, women, and children of all ages who need any kind of athletic shoes
reach shoppers who are ready to buy right away
increase brand awareness
sell high-end running shoes to competitive marathon runners
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. With custom affinity audiences, the shoe company can define this audience by: using free form interests entered as keywords; and/or using URLs as a proxy for interest bundles.
Read more here: https://support.google.com/adwords/answer/2497941?hl=en