From RDFaWiki

Authoring Human Readable RDF Vocabularies

It is considered a best practice to ensure that your RDF vocabularies are both machine and human readable. Using RDFa, you can not only express RDF triples for machines, but create a pleasant viewing experience for humans reading about your vocabulary.

Example

To study a live example of a vocabulary that uses RDFa so that both machines and humans can read it, check out the Media RDF Vocabulary.

Reasoning

Usually, the first URL that an RDFa author visits when learning about a new vocabulary is the prefix URL listed in an XHTML+RDFa document. For example:

<div xmlns:media="http://purl.org/media">

If someone saw the prefix and resulting URL in an XHTML document, they would cut and paste the URL into their web browser and visit the page. If you are the vocabulary author, it is in your best interest to make sure that they can learn as much as they can about your vocabulary at that URL. This can be achieved by marking up your vocabulary page in RDFa such that the page is human readable and that an RDFa parser will generate triples that are valid RDF vocabulary triples.