There are three basic reasons why a roasted coffee should be keep in a tightly capped jar.
1) Rancidity. Improperly capped jar permits the entry of oxygen. Oxygen plus the ground coffee equals rancidity. The rancid aroma and taste can be noticed in just few days.
2) Escape of volatiles. Some customers are snipping the coffee package before making a buying decision. Most of them are delighted if the package has a strong coffee smell.
Coffee package emitting strong coffee aroma is improperly packed. Improperly sealed or wrong choice of packaging material. Volatile aroma should not be coming out. It should have strong coffee smell when opened.
3) Absorption of foreign odors. Medium to dark roasted coffee have the same characteristics to activated carbon. Coffee are partially charred food. A coffee stored near a detergent powder will have a detergent aroma.
On the contrary, it can be used to absorb foul odors, like the stink smell of refrigerator. It can be used to purposely removed the paint smell of hands. Rub unused coffee on hands and the paint odor will go away. The hand will smell like coffee though.