Observing at different wavelengths will highlight different features - both within the atmosphere, but also on the surface. It is usually advantageous to observe in as many relevant wavelength windows as possible, thereby collecting data linking different physical or chemical processes to the same point in time or space.
For the specific wavelengths mentioned:
0.5 μm: useful for daytime cloud mapping and calculation of albedo.
Using just this band we can determine the difference between thick clouds, thin clouds, and water or vegetation. However, with 0.5 μm, we can't differentiate between thick clouds and land.
10 μm: useful for cloud mapping 24/7. Measures the radiation reflected from the highest altitude feature. Using 10 μm we can differentiate between high clouds (cold tops), cloud tops at 'middle' altitudes and (warm) land and sea surfaces.
20 μm: near frequencies of water absorption. Similar to using a ~5μm band (not discussed here), except the focus is on the lowest layers of the atmosphere. NOTE: a typical cloud droplet (not raindrop) is about 20 μm across.