When you are writing code for an Office managed add-in, you can use the Open XML SDK to manipulate the current document in a whole variety of ways. This is a very powerful technique, which in some circumstances can give your application much better performance. This is the key point behind the paper that Anil Kumar, Ansari M, Sarang Datye, and Sunil Kumar wrote, Increasing Performance of Word Automation for large amount of data using Open Xml SDK. In that article, the authors provide the code to do exactly this. Once you understand the mechanics of using the Open XML SDK from within Office automation, it opens up a set of scenarios that would be very difficult to otherwise implement because of performance limitations.