The car wash in my home town of Birmingham, MI has an expression - "A clean car drives better." While we might doubt the veracity of the claim in its literal sense, we have all felt the sensation of driving away from a car wash, detailing, or oil change; the car just feels so much better.
The same holds true for the marketability of a house. A clean house lives better - sells better too. Certainly a clean house and a dirty one have the same objective market value (if such a thing truly exists). A clean house just shows so much better.
Prospective buyers like to, consciously or subconsciously, picture themselves living in "their" new home. They mentally arrange their furniture, hang their pictures, and make their first dinner in the new kitchen. When they see hairs on the bathroom sink (or elsewhere), dust on entertainment center, or smudges on the kitchen counter, they no longer want to live there. Sure, all of our homes get dirtier than we like from time-to-time. We get busy or distracted, or we'll "do it tomorrow". But who wants to move into an already dirty home?! Not many.
Rationally speaking, when investing $200,000, a half million, or even several million dollars on a new home, the immediate cleanliness of it shouldn't really factor in. For $100 you can hire someone to scrub the place from floor to ceiling. What's $100? Well it's about 0.025% of the average Chicago purchase price.
It's also often the difference between getting a second showing on your home and not.