An advertiser purchasing display advertising through other ad networks can still benefit from using AdWords display ads because:
- The ad can appear multiple times on a page to reinforce the advertiser’s message
- She can fill in the gaps of niche and mass-marketed sites using AdWords targeting options
- She can infinitely loop animated image ads
- The ad will show on all ad networks and will reinforce the advertiser’s message
Explanation: The Google Display Network is used by thousands of advertisers to reach users on hundreds of thousands of websites and apps across all publisher categories, from large, well-known sites to niche sites and audiences.
Read more here: https://support.google.com/partners/answer/2740585