Why is it necessary for a constitution to place limitations on the rulers? Can there be a constitution that gives no power at all to the citizens?

It is necessary for a constitution to place limitations on the rulers because absence of limitations would lead to authoritarian form of government that threatens human rights. There cannot be any constitution that gives no power at all to the citizens as they make up the fabric of the state. The institutions of the state that are meant to serve the citizens would end up denying them their rights in such a scenario.