Basically , if full auto aircon is selected it will reach the chosen cabin temp as fast as possible , either cold or warm. On ECO , the time taken 2 do the same is longer. So on a summer day with a high cabin temp after been parked u will want the aircon 2 chill asap , but in normal use the extra time taken 2 control cabin temp is acceptable. Benefit is as previously stated , costs less in fuel.