People don't consciously notice them unless they're disgusting or enticing, and writers notice them even less often, if most novels and stories are studied, but smell is one of the strongest memory-tied senses. Smells evoke memories and emotions more effectively than sound or touch, and that's why good fiction uses the reader's nose.
Smell can set a scene as effectively as any other descriptive method, and do it in a non-intrusive way.
The hotel room was tattered; stained bedsheets, torn lampshades and a carpet splotched with unidentified stains.
The hotel room was old, beaten-up, and full of warm, sour air.
The second sentence is shorter, cleaner and involves three senses, while the first relies on visual cues alone. We've all smelled a bad hotel room. The second sentence recalls it to our mind and we build the stains and tatters and crooked pictures on the walls.
Remember: Your best canvas lies in the reader's mind. Paint there. Use their senses to help you.