When you add (or update) the Readership Audience for a specific document with a SharePoint Group or Audience you will create a new version of the document when it is saved. We have designed DocRead this way because we found that storing (and versioning) the audience as document metadata has these advantages..

1. Each change in audience is recorded in the documents version history – great for auditing reasons. (Using this and "modified by" you can see who changed what).
2. As the audience is stored as metadata, this makes integration with workflow easier. For example, if you want to change a documents readership from a small group to "All company users" this really needs to passed through an approval workflow, rather than just being allowed to do it.
3. It makes scripting easier – for example, it's quite easy to put together some Powershell to programmatically upload and set the Required Audiences. (see here as an example : http://www.collaboris.com/blogs/collaboris-blog/mark-jones/2012/02/14/update-multiple-documents-with-powershell)
4. Existing customers find it very intuitive and easy to use. If we moved it somewhere else we'd need to think carefully about where would be best without making it confusing.
5. If a document (with an assigned DocRead audience) is deleted (or moved), then if you restore it, the information relating to the audience is also restored.