Smell is one of the most powerful of all our senses. The pity is that most writers spend almost no time on the smells of their worlds.
While sight is far more important to humans than smell, smell has the ability to evoke emotions and memories like no other sense. If you can convey the smell of a thing to your readers you can make it come alive in their minds.
This isn't easy, which may be why it is so effective. Our vocabulary of smell words is limited and most of us have more trouble "visualizing" a smell than a sight or a sound.
And of course as with any sense impression the more concrete and specific the language the more vividly the impression is conveyed. In the case of smells this usually means a lot of hard work.
But then hard work is what makes good writing in general.