Light bulbs are usually filled with a mixture of gasses, selected to help retard the evaporation of the filament and to minimize heat loss from the filament. In any case, the goal of both strategies is the same: to allow the filament to have a reasonably long life when burning at the highest possible temperature, burning more brightly and more efficiently.

Halogen, on the other hand, isn't a particular element or chemical. Argon works OK for this, krypton works better, and xenon better still. The better gasses are more expensive, of course. The halogen gas in the bulb can actually bond temporarily with evaporated filament metal, and redeposit the metal back on the filament. This extends the filament life and keeps the bulb's glass free of the filament deposits that make other bulbs turn gray as they age.