We humans don't really appreciate the sense of smell.
Out of all of our special senses it is probably the one that we can most easily live without. In fact lots of people have lost their sense of smell for one reason or another, and yet relatively few of them complain about it to their doctor. They may have to keep more of an eye on the toast than usual and will have problems detecting gas leaks, but they live pretty normal lives. You can even imagine instances when it could be a positive advantage not to have a sense of smell, such travelling on a rush hour tube train or when changing a baby's nappy! However, we are something of an exception in the animal world where the sense of smell is often vital for survival. Smell helps in recognising food and whether it's good to eat before it gets anywhere near the taste receptors in the mouth. For instance young animals can develop a preference for the smell of food that has been eaten by their mother, even before they have been born. Similarly, rats can learn about whether a new food is good to eat by its smell on breath of other rats. The sense of smell is also used by many animals to track their prey, or to warn them about predators.
Some animals can use odours to navigate around their environment. For example, salmon learn the smell of the stream in which they grew up. When they come back, after spending two years at sea, they can recognise the smell so that they can find their way back to the same stream to breed. But probably the most important role of smell is in social communication. For instance, when a male mouse meets a novel female mouse, he will spend a long time sniffing her body, and especially her hind end. This not only tells him if she's ready to mate, but also by sniffing different females the male can chose to mate with those that are less genetically related to him.
Whereas the smell of a female silkmoth will attract males from over two kilometres away, this would obviously cause havoc if it occurred in humans! Human social interactions are just too complicated to be dictated by smells, and need more flexibility than the simple "come hither" signals used by many animals. Nevertheless, although smells are much less important for humans, we shouldn't underestimate them. The subtle influences they have over our behaviour are shown by the vast amounts we spend each year on perfumes, fragrances and flavourings - or is it just clever marketing?