Coffee (and other dry perishables) go stale for two primary reasons: exposure to oxygen and humidity. Atmos and other containers on the market solve the oxygen problem by removing excess air. Atmos does so by lowering the pressure whereas other devices remove excess air by shrinking the container. They're both effectively doing the same thing, but what makes Atmos unique is that it reduces both oxygen and humidity, subsequently extending the life of your coffee by up to 50%.
So, let's say your coffee has a 10-day window where it is in its prime (beginning 3-5 days off the roast date). Storing your coffee in Atmos will effectively give you a 15-day prime window. That's not to say your coffee isn't good after this timeframe, just that the unique flavour profile will begin to fade each day thereafter. On a side note, coffee that is more than 20 days off roast is the prime candidate for a killer cold brew, so don't let it go to waste if you find it no longer yields the perfect hot cup.