It's more than just being safe, it's the LAW in Iowa.
According to Iowa Code 321.384:
Every motor vehicle upon a highway within the state, at any time from sunset to sunrise, and at such other times when conditions such as fog, snow, sleet, or rain provide insufficient lighting to render clearly discernible persons and vehicles on the highway at a distance of five hundred feet ahead, shall display
Meteorologist Derek Kevra of Fox 2 News in Detroit appears to be fed up with meeting cars on the roads that are covered in snow.
He posted the video on his YouTube page along with a two sentence description:
It must be a hard concept because I see people driving around with snow covering every window...