Wet pavements are far more slippery than dry pavement. When the road is wet, it is much more difficult to hold the road on curves and it takes a longer distance to stop. Be especially careful at the beginning of a light rain; the mixture of oil and water on the pavement is particularly dangerous.
The oils and other vehicle fluids have not been washed away yet as they would after a rain of several hours. Similarly, watch out for wet leaves on pavement in the autumn.