Switched to plastic risers after my corks got moldy, and @diana_knight is right about the airflow. I mean, the corks worked for a season but started to decompose in the dampness, which attracted bugs. Now I use those little green plastic feet, and they keep the pots elevated without any blocking, plus they're easy to clean. Idk, maybe it's just me, but seeing the water actually drain away makes a huge difference for root health.