What does an ad server like Zedo do?

We have been running a successful Google Adwords/Microsoft Advertising account for quite some time now, and now we are looking to branch out into some display networks with some higher traffic (i.e. AdBlade, AdBrite, Pulse360, etc.) We have an herbal supplement and are looking to advertise using an article style landing page, but certainly not a "farticle" like those bogus landing pages that **** berry products used.

But I have 2 questions, since this is uncharted territory we're going into now:

1.) What are some more popular display network sites out there besides the ones that I mentioned that I might use to promise an article-style landing page? Are PPV networks good for this sort of thing?

2.) What does an ad server like Zedo do? At first I thought it was its own ad network, as I noticed that one of my competitors uses them. However, now I realize that they don't actually own any ad placements but rather serve your ads to other websites (or something like that?). Why would I need Zedo as opposed to using these ad networks' own platforms? What is the point of using Zedo?