MIDI only transmits on 0-127 for everything, velocity, CC numbers, etc. etc. etc.
So it gets really inaccurate for minute changes, like pitch control. If a pitch fader can only transmit on 0-127. When you're dealing with 10% that doesn't mix very well, neither does 8%. But 14-bit MIDI transmits a LOT more detailed. I don't know the numbers, but essentially it allows a lot more information to transmit to allow a lot more accurate info.