One of the most important (if not the most important) to appretiate tea. You can learn all about the sense of smell on the net. Smell 101 provides a good introduction to how we smell.
There is one experience you can do yourself: close your nose and then drink a cup of tea. What do you notice? It becomes quite obvious how much is lost when we can't smell! There are thousands of different smells our nose is able to distinguish and identify! Compare it with the sense of taste of the mouth (located mostly on the tongue) and you'll feel how powerfull our nose is.
Like all senses, you need to focus your attention on to increase your skills. Tea tasting is a good way to explore the almost limitless possibilities of your nose.