Mounting Tolumnia... A Question

[WaterWitchin]

I am determined to have some mounted tolumnia. A part of this year's goals for me. Am I good at raising tolumnia? Ummmm, not really. Thus I keep searching for something that will work for both them, and for me. A method where they will thrive, not just survive.

I've read thru numerous threads regarding mounting them, and most talk about adding the water-retention of "something" either over or under the mount.

Here's the question... I don't understand "why" the spagnum moss or coco-fiber, etc, either under or over roots on a tolumnia mount. It seems counter to its requirements.

Would someone be willing to explain the "why" of the water-retention material? I would truly appreciate it before I proceed.