Those files are there to allow users to update their PAI AI models as needed.  PAI is in the process of updating their older models with newer ones that use less polygons and have more levels of detail.  In addition those files provide for updated FDE's or flight dynamics which also get updated as they go.

Several of their airline packages are years old and therefore the models themselves aren't as good as the new ones they are putting out.  This way instead of updating the whole package you can just update the individual aircraft models that you may need.