Let’s start from an assumption: if your message/product is not seen cannot be taken into consideration. If it is seen, but is not recognized, it is not effective. If it is recognized, but cannot catch the attention, will not be chosen.
We are used to assess the efficacy of a banner, of a website on the basis of visitors or impressions. But in a context of over-stimulation, where there is a lot to see but little time to do it, being able to catch the attention and generate interest in the user, is becoming increasingly difficult: on third of advertisements is not seen, but this may also happen for important elements of your product or website (call to action, logo, price, etc).
While viewability is until now estimated, the attention detected with the eye tracking technology is an objective KPI.