for me its the completness and flexibility. If i want to play in a band i can reduce the arranger to a straight synth/piano instantly. If i am backing a choir i can play a full accompanimant instantly or remix entire songs on the spot live and try out new arrangments in seconds. And iof i just want to jam by myself i can do that just as easily. It is the complete instrument. some people get really tunnel visioned about arrangers but to me its a keyboard and then some !