Soz, missed your question. Yeah, just to stop the compost sucking up all the water and rotting the roots. As someone who has form for overwatering, these are great. Some roots still exit the bottom and fill the trays but not to the detriment of the overall health.
Interesting, what's the logic to them not being sat in water? Is it that the soil risks gets waterlogged and then drowns/suffocates the roots?
(I ask because we have a gro-bag reservoir and the roots inevitably follow the wicking fabric and then spread out into the reservoir)