Juggling helps us develop inter-limb coordination, ambidexterity, fine-motor and gross-motor control, rhythmic timing, body-mind awareness, mental focus, spacial awareness, self-discipline, proprioception...... basically all kinds of crazy power.
I'll leave it up to your imagination as to how those qualities could relate to piano-playing

None of my students practice enough, though. Juggling is frowned upon in certain circles:(
because it is so AWESOME!