I think it was used as a high-voltage clipper... sort of a high-voltage
zener diode. During normal operation it's never supposed to light up. I
often saw those on neck boards of old monochrome video monitors. Only once
did I ever see one light up and that was due to picture tube failure (the
precise details of which I can't remember now). The firing voltage of neon
bulbs is pretty high... at least 50 volts, some go as high as 90 volts.

BTW: If the bulb is placed across DC, only one of the interior electrodes
will light up. If the bulb is placed across AC, then both internal
electrodes will light up.