Explanation

Argon is a chemically inert noble gas, so it does not react with the hot tungsten filament. By filling the bulb, it suppresses oxidation and slows the evaporation of tungsten, which is what thins and eventually breaks filaments. A longer-lived filament means a longer-lived bulb. Argon does not colour the emitted light, reduce power consumption or cut the cost of the bulb. Hence it is used to increase the life of the bulb.