There can be times when the channel is idle, when a node has a frame to send, but is prevented from doing so by the medium access protoc

There can be simultaneous transmissions resulting in collisions.

Channel partitioning protocols can achieve $100\%$ channel utilization, in the case that all nodes always have frames to send.

Channel partitioning protocol can achieve $100\%$ utilization, in the case that there is only one node that always has frames to send