For some reason, the accepted visual language of the movies means that all streets are wet at night. Anywhere, anytime, any place. Night = wet street.
Which is fine. These visual cues help movie viewers to figure out time and place without tiresome dialogue about it (“We’re going to go outside at night!” “Swell!”).
To achieve wet streets takes water. On location, it’s water trucks, but on studio lots they use hoses to spray the street with water right before we shoot.