What is the inverter (black box) used for? Do I need it?

It is a DC to AC inverter. Yes, you do need it for gauge dial backlighting. It is not necessary for gauge operation. The dial lighting is accomplished via an electroluminescence panel behind the dial. This panel glows when it is subject to around 100 volts AC. The DC to AC inverter takes 12 volts DC and creates the needed 100 volts AC to power the dial lighting. Without it connected, the dial will not illuminate. Normally customers will wire the black wire from the "black box" (inverter) to a chassis ground and the white wire to the same location as the white wire on the gauge power distribution cable (red/black/white ribbon).