A bit of air in the jar will help the tobacco develop it's flavor and aroma, and allow it to continue improving with age. To test this for yourself take a freshly opened tin or a newly received bulk blend then give the tobacco a sniff, smoke a pipe of it, and note the aroma of the tobacco and flavor of the smoke. Now seal said tobacco up in a Ball jar with some air space at the top (at least half an inch, but even leaving the jar just half full of tobacco and half full of air is perfectly fine) and don't open it for 2 weeks.
When you come back and open the jar again to smoke some after 2 weeks take note of the aroma of the tobacco and the flavor of the smoke. You'll likely find both to be significantly more intense and flavorful than they were before you jarred it up!
If you have enough of the same blend to do so you can also perform this experiment with a second jar that you pack tight all the way to the top to push all the air out, and you'll likely find it that after being sealed up for 2 weeks it won't be quite as pleasant smelling or flavorful as the jar that you left some air in.
