This command exposes the API for freezing IO. Snap software for SANs uses this to stop IO and get a disk mark where they can start copying data. In talking with a few people, some snap software accesses the API, some calls the DBCC command.
If you read the reference, you can play with the commands and test how disk access occurs in SQL Server. It's undocumented as it's a troubleshooting/experiment command, not a useful one in daily work.