When incandescent light bulbs are manufactured, they are filled with an inert gas to insulate the filament and prevent its sublimation. The pressure inside a freshly manufactured light bulb is something like 80% of the normal atmospheric pressure. The gases normally used are argon and a little bit of nitrogen.
Why not fill them with gas at normal atmospheric pressure?
Best Answer
Light filaments are mainly tungsten, which oxygen corrodes, so reducing the lifetime of the bulb. Argon reduces this corrosion and basically, you only need a small amount, so it keeps the cost down by using the minimum amount.
This duplicate Pressure inside a light bulb seems to answer the rest of your question.
You want to have the bulb at atmospheric pressure when it's on, and hot, so having it at 80 percent when it's off, means a bulb turned on will then go to normal atmospheric pressure.
If you filled them at normal pressure when cold, they are more likely to fail or explode when heated up.