Why is an electric bulb evacuated?

An electric bulb, particularly the incandescent bulb, is evacuated to prevent the reaction of air with the tungsten filament in the bulb which would appreciably shorten its lifespan. If the filament were exposed to air, there would be rapid oxidation of the filament. To prevent such an event, the filament is housed inside a glass bulb that is either evacuated or more frequently filled with some low-pressure inert gas like nitrogen, argon, or krypton. The inert gas slows down the evaporation of the filament material but also dissipates away some heat.