The content_purge_boundary_date of each stream is the earliest date of content still available on the local disc of the Box:
For example, this value means that there is some content from 31 days ago that can still be retrieved from Box:
content_purge_boundary_date:7/15/2019 12:01:12 PM (31.3d ago)
The primary reason to know the content_purge_boundary_date is to verify that the the Box's disk capacity is sufficient for Lazy Indexing and Query Match Indexing. For example, if you're uploading Events automatically only when there is a human or vehicle present during nighttime hours, and you still want to perform ad hoc forensic searches for other Events during the daytime, then the content_purge_boundary_date tells you how long you have to realize the need to retrieve that daytime video that's not yet in the cloud.