Evidence for using capacitor-input filters?
I'm not going to explain the pi configuration, but you can understand much of its function when you look at the individual components.
The impedance of a capacitor is inversely proportional with frequency: the higher the frequency, the more it conducts. So high frequency disturbances are shortcut to ground. Also, the impedance of the coil is proportional with frequency: the higher the frequency the less it conducts. So the high frequency disturbances which haven't been filtered away by the capacitor will be blocked by the coil.