^^^
Or the Cliff Notes version.....it allows someone with a '11 or '12 which lacked the incredibly handy SOC% display the ability to see SOC.
Note the DD displays more than SOC but it's mainly what I use mine for, it also displays GIDs and battery SOH and a few other things I'm sure that link explains, think of it as a standalone LeafSpy display without using your smart phone.
Personally I wouldn't use an '11 or '12 Leaf without a DD or running LeafSpy with an OBDII adapter everytime I drove. The GOM is woefully inaccurate. Very optimistic with a full battery and quickly turns to pessimistic as you approach mid-SOC or lower.