Imagine you do not have a single cent to spend on a website but you still have a service that you'd like to offer.
What would you do? Most likely you would build a web 2.0 in order to raise awareness for your business.
Web 2.0s work in the same way as a normal domain, only difference is that they are part of an already existing, and most of the time established, domain.
So in short, they cannot only bring you traffic but they could also act as the main site of your business. If you think smart, you will start treating the web 2.0s used in your Tier1 in a different form and try for them to be as "big" as possible as they can certainly bring a lot, and not just traffic