John, with the brakes you press the pedal to load the pads against the discs, when you release there is no pressure in the system so the pads can move apart due to disc flexure and play etc. The residual pressure valve keeps them in light contact to ensure there is no gap to take up and that there is little or no pedal travel before re-establishing contact the next time you brake.
By contrast, with the clutch you press the pedal to oppose clutch springs and remove load from the clutch facing. When you release the clutch, the springs put the load back on, so residual pressure in the system would be counterproductive.
Dave