Gas-filled binoculars
The purpose of filling binoculars with gas (usually nitrogen) is to prevent the binoculars from condensation and fogging up.
The chance of condensation is greatest when temperatures fluctuate a lot (e.g. when bringing the binoculars indoors from outdoors during winter).
When condensation occurs, the image contrast decreases. After all, you are looking through the 'fog'. The moisture can also cause mould to form on your lenses and this often heralds the beginning of the end of your binoculars.
