They are relatively bright and have a low power consumption. Some older calculators used them before LCDs became popular. A plea went out on the PIC list and was soon answered by Kalle Pihlajasaari with a few details on VFDs. The filament requires about 40-120 mA at a few volts. Too high a voltage will make the filament glow red or even fail.
Most displays have more than one filament wire in parallel.
The filament is held at close to 0V. A high signal, usually 12V, on the grid and anode lights up the corresponding segments. The grid drives the digit and the anode drives the segments. The Futaba web site was no help and a search on the web turned up thousands of references to Futaba servos and little else. The VFD seems to have been made for an oven. The leading digit has an AUTO symbol next to it.