Adding hydraulic resistance beneath an espresso puck reduces channeling by slowing the flow of water and making it more even across the coffee bed.

I now suspect that adding a source of hydraulic resistance under the puck that is non-negligible compared with the espresso puck itself is desirable to reduce the impact of channels because it slows down the flow of water where it would tend to become otherwise too fast and as a consequence less even across the surface of the coffee bed.