For the details, check out this article.
But the last paragraph is the essence:
In other words, WHO (an organization with dubious reputation lately) is going to decide WHO (pun intended!) and WHEN will freely travel within EU (and in other parts of the words possibly). New pandemic? Well, you need new vaccines, new tests, new stuff. Another mark on your certificate. The national passports will become irrelevant since the new one will be the starting point of any travel.
The market for the vaccines is huge and money is to be made, but they need to keep repeating (every year?). This "passport" just might come in handy.
