Inside the vintage Xerox Alto's display, a tiny lightbulb keeps it working
Using various interface-ish components only for their electrical parameters is neat trick that is very common in certain applications. In this case the lightbulb is slightly weird but probably serves to shape the 600V pulses in some way and also to protect the switching transistor should the grid outputs get shorted to ground. Looking on the schematic there are also neon lamps across the outputs to protect the CRT from overvoltage (which you will find in essentially any modern CRT monitor).Another such neat trick is in most simple photographic flashes: the neon bulb which indicates that flash is ready also provides feedback to the flyback charging supply by increasing it's load when correct voltage is reached.And then there is typical CRT degaus circuit, which produces the required damped oscillation waveform by running mains voltage throught two thermally coupled thermistors.Edit: Horizontal deflection circuits of CRT TVs and monitors are usually full of neat tricks: shifted hysteresis inductors, deriving various voltages from the flyback and output multiplier, synchronisation of main SMPS switching frequency to horizontal retrace... One can view CRT VGA monitor as one big switchmode power supply with video amplifier bolted onto it (and many CRT monitors are actually mechanically built this way, with RGB video going directly from the captive VGA cable to amplifier on the neck board)