Why are incandescent lamps with tungsten filaments filled with an inert gas such as argon?
Because argon is an inert gas, that does not readily react with the hot tungsten filament............
The tungsten filament becomes white hot when the circuit is made and the light is illuminated. As an inert gas, argon does not readily ionize, and protects the filament when a current is passing thru it. I once put this question to an engineer in a lamp factory, and he told me that ideally, a light bulb should be under vacuum, so that there was NO gas inside the bulb to ionize. Alas, it is easier to fill the bulb with an overpressure of inert gas, than it is to evacuate it then seal it.
When you run a inert gas glove box, often you can drill a hole in an ordinary light bulb (which is not a trivial thing to do), take it into the box, and connect the power. While the light bulb still shines the atmosphere is good.