How does an extension work on a Display ad?
It extends the ad below the fold
It adds extra information, like a location or phone number
It extends the length of the ad placement
It adds a testimonial
AdWords shows one or more extensions with your ad when it calculates that the extension (or combination of extensions) will improve your performance, and when your Ad Rank is high enough for it to appear. Adding an extension won’t guarantee that it will show with your ad, but you can keep track of when your extensions are appearing on the Ad extensions tab. Since AdWords requires a minimum Ad Rank (factoring in your extensions) before showing extensions, you may need to increase your bid or your ad quality (or both) in order for your extensions to show.
By adding extra pieces of information about the business to their ads, they’re giving customers more reasons to click.
Read more here: https://support.google.com/adwords/answer/2375499?hl=en