You have the notes in order on a keyboard, as in... lowest on the left, highest on the right, one after another. On the MPC this isn't the case.
Biggest difference is playing chords. Let's say you are going to play a low chord with your left hand and a higher one with your right hand on the right side of the keyboard, at the SAME TIME. How are you going to do this on an mpc?
Besides, completely different feeling to play a keyboard compared to playing MPC pads?
In short: It's a HUGE difference. I thought that would be pretty obvious...
Also, you most likely have a pitch bend and mod wheel on a master keyboard. Sure, you can emulate that with sliders on the mpc, but with a master keyboard it's just there and works for every key.
Your question is like asking: "What is the advantage of playing a real guitar compared to triggering guitar sounds with MPC pads?".

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