Targeting by topic is a good strategy if your client wants to:
- drive sales on his website
- reach a specific audience
- control where his ads appear on the Display Network
- actively manage his budget because he has strict cost-per-acquisition goals
Explanation: Topic targeting allows your ads to be eligible to appear on any pages on the Display Network that have content related to your selected topics. As content across the web changes over time, the pages on which your ads appear can change with it. To display your ads on those pages, simply select one or more topics that you find relevant for your ads.
Read more here: https://support.google.com/adwords/answer/2497832?hl=en