Remember this is a complain about someone who plays piano
is A because must you play the piano at such a later hour?is a question for John
.你真的需要这么晚了来弹钢琴吗?
B need you play the the piano at such a later hour?is not a question,is more like that someone wants john to continue doing..but that's not what he wants..he wants john to stop...more,you asking him..why must you play the piano at such later hour?not why need you play the piano at such later hour.need doesn't make sense in this sentence...try to put "why” in front of the sentence and read it.makes more sense.