DOAP stands for "Description of a Project" and you can find out everything there is to know about DOAP in its homepage and in this article.

The Semantic Web provides mechanisms to process data provided in a form that is easily processed by machines. Thus, a DOAP is a machine readable document which facilitates projects research: it becomes much easier to seek information in the mass of data of the Web, since the data have a given foreseeable format.

That's a very good question! The answer is that generating a DOAP file should take no effort if you are using Maven DOAP Plugin and it helps disseminate project information which can only be a good thing. Cataloging tools like SWiK or like DoapStore can benefit from you generating DOAP files and that can also only be a good thing. Even so, it is still important to spread as much information about projects around as possible so there is no downside to creating DOAP files.

Maven DOAP plugin has generated a DOAP file, what's next? See DOAP in Use part.