Rich Media typically costs the client more 💰 than standard media.  They are basically buying a better placement on the publisher (the site). Or, they are agreeing to pay for more bandwidth or k-weight.

The vendor actually creates the bit of embed code (the tag) and launches the advertisement. Vendors have relationships with different publishers (sites) and are sometimes chosen based on this.  Another competitive aspect is their ability to provide development teams with components that speed up production time. And a critical role that they play is ensuring that the unit runs in all browsers.