A sunny day in Belgium is as weird as a rainy day in Spain, so you get used to see the beauty on it.
Going out for a photography walk while raining provides much more opportunities like reflections in the puddles, drops in the trees, rainbows… And the clouds look just amazing in W/B with a high contrast.
But it gives you privacy as well, and that is perfect for social photography. People is busy trying to not get wet and fighting with the umbrellas, so you are completely free to catch all those moments from the distance.
A picture trough the train window in the Gare du Midi, a cyclist in the Place Royale or a woman fighting with her umbrella in Porte de Namur are some good examples.