Both have the goal of to make it easy to get newer software, or software from other sources, within the same management interface, and without breaking all the other packages of the distro. But transactional packaging systems does get to those goals, while PPAs just maybe get there, for a time, but always turn-out breaking something.

But if you are using PPAs to create different lists of released software, so you can keep you stable software while everybody else wants to move to something newer, transactional packaging does not have that goal. Although it makes the job of maintaining such PPA much easier.