Gas to expel the 'cream' Without it , nothing would come out when you press the button. This gas is responsible for binding to the milk fats when you prepare the whipped cream with a whisk - 80% of the atmosphere is nitrogen - and in a propellant canister, the nitrous oxide acts to make the cream 'thick' .
If you have a restaurant style dispenser which uses replacable cartridges, if you have the CO2 cartridges - like for sodastreams - these don't make whipped cream as the fats are not able to form solids in the cream as the gas is no use, and you end up with carbonated cream instead.
Actually its 'laughing gas', so if you wanted to have some fun . . . . .