This requirement helps motorists see other vehicles during this inclement weather. The State law is below for a reference.
RSMO 307.020 - when lighted lamps are needed
"When lighted lamps are required" means at any time from a half-hour after sunset to a half-hour before sunrise and at any other time when there is not sufficient light to render clearly discernible persons and vehicles on the highway at a distance of five hundred feet ahead.
Lighted lamps shall also be required any time the weather conditions require usage of the motor vehicle's windshield wipers to operate the vehicle in a careful and prudent manner as defined in section 304.012. The provisions of this section shall be interpreted to require lighted lamps during periods of fog even if usage of the windshield wipers is not necessary to operate the vehicle in a careful and prudent manner."