It does seem confusing. This is referring to changes which are now in beta. These do seem to be big changes, meant to be a different way of working with your entire repo.
AFAIK, projects has always been a sort of bolted on project management app. These newest changes in beta seem to be a quicker way to manage a high load of activity on a repo.
The text in my GH present the product as "Built like a spreadsheet, project tables give you a live canvas to filter, sort, and group issues and pull requests."
The spreadsheet idea is the key I think. Imagine that a spreadsheet like interface would be easier to managing a high volume of activity, rather than a click to form view.
I was definitely using some project management tool within Github 3 years ago.