I watched The Insane Engineering of the 787 by Real Engineering. He just gave short detail about the HLFC(Hybrid Laminar Flow Control) panel and mentioned their purpose isn't available online. I found the following from flight global:
HLFC surfaces are designed to suck in the surface airflow, allowing the boundary layer to remain attached, moving the onset from laminar to turbulent flow further back along the surface, reducing overall drag.
So why they aren't mounted under the pressure side of the wing same as on the horizontal stabilizer? It would decrease the drag significantly. My other concern, the flow is more turbulent on the suction side. Isn't it reasonable to mount an HLFC panel on the suction side rather than the pressure side of control surfaces?