There are some reviewers like Regulatory who need to make dozens of comments and have offline collaboration on a routed PDF before adding comments to the proof.
While they like PHQ (Workfront Proof) by-and-large, in our old system it was more efficient to download the PDF, collaborate and make comments, and upload it via Adobe's FDF file functionality (where offline comments can be merged with a shared online proof).
So whereas in our old system they could upload the final comments via FDF in 15 minutes, it takes an hour to completely retype and re-markup online. That's costing us valuable time and money.