The main purpose of inert gas in a light bulb is to prevent the tungsten filament of the light bulb from decaying as the light bulb heated to very high temperature due to the heat generated by the the tungsten filament.
Suppose, a light bulb having air inside the glass, it would not glow as tungsten of filament would react with the oxygen present in the air resulting burn out the tungsten filament during heating process. Since inert gas do not react with tungsten, light bulbs are filled with inert gases like argon and nitrogen. Inert gas bounces tungsten atom back towards the filament to prevent it from spreading and coating inside the bulb.
Conclusion
Hope, you have clearly understand the reason why inert gas is present inside the light bulb. Male sure to leave your opinion and suggestion about this post in the comment section below.