Head's up that the source goes into some detail and is a miserable, sickening read.

Per the source (https://cdn.arstechnica.net/wp-content/uploads/2017/02/fedsr...), the prosecutors already have a case based on checksums of the media that the defendant had downloaded, and per the logs, stored on his external hard drive. I imagine that the prosecution wants the media so they can perform harm reduction services for the identified, affected children and/or improve their data for going after other/future child abusers.