FB - the Snellen charts visible inside the Hoffman Box
As AAKEE says, it is the ability to compare goggles to a standard to know which ones work best that is the one big advantage.
The other big plus is that you can teach people how to set their goggles up properly from the outset. When I was a Squadron Training Officer, I used to pick up pilots goggles that they had 'set up' and check them on the Hoffman box - the number that had used the rear, dioptre adjustments to get a good picture instead of the front adjustments was surprising and they were giving themselves eyestrain trying to get a good focus on an image that wasn't properly focused in the first place.
I know I have mine set up properly if I end up with the sharpest image I can get - usually 20/35 - and the dioptre scales at the back match my eyesight prescription for reading glasses - between +1.0 and +1.5.
The only slight downside is that the Hoffman Box gives you a set up with the focus on infinity which isn't much use for helicopter operations so you still make a real world adjustment once in the cockpit or on dispersal - but you are doing it from a known and correct setting.