Oxygen is needed for life and change. Mold needs oxygen to survive and CBN needs oxygen to change THC. Both of these degrade or ruin the potency and viability of your cannabis. Packaging cannabis can overcome these problems with a process called nitrogen flushing.
Nitrogen flushing occurs right before the packaging is sealed. The process forces oxygen out and replaces it with nitrogen. Several benefits are gained by this extra step.
As already suggested, reducing oxygen preserves the THC level as well as hinders mold. There is another protection benefit of Nitrogen.
Nitrogen flushing causes there to be more air pressure inside the package than out. This positive pressure cushions the nugs lessening breakage during transit and handling. Lastly, while not a quality control, the nitrogen masks the smell of cannabis allowing you more privacy.