The water usually brings a small amount of dirt with it, so it's not clean and clear. The drainage plate reduces the risk of water damage and staining by keeping the water directly under the pot and off the surface below. These plates are easy to remove are the best for drainage.
These planter plates hold the water too close to the soil, which sometimes defeats the purpose of allowing drainage. If you see water in the plate after watering your plant, remove the plate and dump the water outside as standing water can be wicked back into the soil and saturate the roots. Replace the plate and leave it under the pot.