Can someone enlighten me as to what this does?

Jaguar Land Rover SDD V134 04 Diagnostic Tool Panasonic CF 29 | eBay

 

This is the factory diagnostic tool. It is by far the BEST tool, however, it says it is V134, which was setup for "online" usage. So in order for this version to work, it has been "cracked". Cracked versions usually don't program well, if at all, and you won't have any access to service bulletins or any other PDFs that are available through the tool.

Just keep in mind, a tool this powerful can do damage to the vehicle if you don't know what you are doing.

Also, it only works on 2006 and later Rovers. For 2005 and earlier, you still need the T4.