When optimizing for the Display Network, a clear call-to-action (CTA) in the ad text is important in order to:
increase overall impressions for the ad
identify the traffic to your website that was generated by AdWords ads
improve the Quality Score of the ad on all Google properties
set expectations for customers who are in various stages of the buying cycle
Be explicit about your call to action, whether a button or somewhere else. This is especially important for image-based ads, because customers may not know what part of the ad is clickable. In some cases, if your ad really blends in well on a website, they may not even realise that your ad is an ad! Don’t make them guess.
Read more here: https://support.google.com/partners/answer/1722134?hl=en-GB