“[Wet streets in movies] look slicker, but there is a practical reason we DPs [directors of photography] often do a “wet down” for nighttime exteriors. When a surface is wet, the diffuse reflections are nearly eliminated; those remaining are the specular reflections, the ones that are the twinkly points of light and the long lines from light sources facing the camera.
Elimination of diffuse reflections removes shadows from the ground plane–shadows from multiple light sources, “flags” and stands. These shadows can be a real time-killer (read cost) to get rid of any other way, and if left there, they simply look terrible and “give away the gag.”
The elimination of diffuse reflections on a wet surface is the reason it’s so difficult to see the stripes on a dark road in the rain. There are no diffuse reflections back to you from your headlights; the only reflections are specular–they bounce off the road away from you, as if the road were a mirror.