MIDI mapping assigns physical controls to software functions: this pad triggers that clip, this fader rides opacity. The protocol dates to 1983 and its 0-to-127 controller values still run most performance rigs, because cheap controllers are everywhere and every tool listens.
VJ practice treats the mapping itself as instrument design: which gestures deserve dedicated hands, what lives on banks, what must never be two button-presses deep during a drop. Muscle memory is the actual product.
The limits, 7-bit resolution and no network, are why OSC exists. In Ableton-centered rigs, parameters exposed to the DAW (as Max for Live devices do) inherit its mapping and automation for free.