According to IAB, publisher signatures allow a buyer to validate some aspects of the bid request and know that it’s trusted from the publisher and key elements of the bid request are unmodified.
Ads.cert is a measure to fight App/Domain fraud; IP fraud; Device ID fraud; Consent fraud; and basic ad format fraud (for example making a display request look like a video request).
Ads.txt, adopted by most of the publishers, was a macro measure to fight unauthorized inventory arbitrage and domain spoofing. With the Ads.cert the authentication goes into a deeper level, where all the bid requests will be encrypted and authenticated.
Publishers will have more control of the ad inventory and higher prices of ad space. Advertisers will buy less ad fraud, resulting in more effective marketing campaigns.