Channel sensing is used in MAC protocols to optimize the Energy Efficiency vs Throughput when RSSI (Read Signal Strength Indicator) is used in any other type of receivers to indicate the level of the signal (usually just before the detector).
To understand the difference from a standard level reading there are a few factors of channel sensing that should take into consideration: carrier sensing (coherently sensing), energy detection (non-coherently sensing) and preamble detection (using a matched filter).