Why Publish D20 Content?
The amount of content released in the System Reference Document is rather extensive. So much so, that the list of things that are not useable is much shorter. See some of the content that's not included here.
Dungeons and Dragons is arguably the best known role-playing game to date, and much of its content (from all editions) is easily recognized by almost anyone in the role-playing community. Publishing D20 based content gives you the ability to use creatures and spells and abilities that people recognize, and can understand. Otherwise, you'll have to create all your own creatures from scratch, and spend time developing and introducing them.
The release of the System Reference Document built a platform so large and so extensive that it's really become the gold standard of role-playing systems. It gives publishers the opportunity to focus on the game setting content, rather than the mechanics.
However, this license is no longer available for use as of 2008. If you were hoping to publish under the D20 license, you are out of luck. However, Pathfinder us a very similar system, and you can publish your content under that licenses the way you could have published D20 content prior to 2008. For more information on the Pathfinder license read here.