Dirty or streaked glass diffuses and dulls natural light entering a room, which shows up clearly in photography as rooms that look flatter and less bright than they actually are in person, even with otherwise good staging and camera work.
This matters more in rooms that rely heavily on natural light as a selling feature, living areas and kitchens in particular tend to be judged partly on brightness and openness, both of which are directly affected by how much light is actually getting through the glass.
A thorough interior and exterior window clean ahead of photography day is a relatively small task with an outsized impact on how bright and inviting the final listing photos read, particularly for properties marketed on natural light as a key feature.
