All laminate flooring is installed with the free floating technique. All laminate flooring is manufactured with a wood fiber core layer that is sensitive to moisture and will expand and contract minutely as the humidity changes. For that reason it is never attached to the subfloor.
Because it is not attached to the subfloor, the surface of the laminate flooring does not lay tight on the subflooring. When an individual steps on the flooring it depresses until it touches the subflooring. Without an underlay there will be an audible click for each step on the laminate flooring. The underlay eliminates this click.
The underlay also must have a vapor barrier to lower the effects of moisture that may rise through the floor after installation.