this is a good question. purely based on what we know so far, my best logical guess is that the texture discs are used to unlock said things, but have the option upon placement to overlay everything in case that's what you wanted to do.
so let's say for example you wanted a nightmare before christmas tree. you would place a normal default tree of your size liking first, then use the editing wand to then 'paint' it the theme that you want. that way instead of cluttering your menu with 100s of trees, it's just the different size and shape trees that can then be skinned to the various themes.
i totally hope that you can use the trees and textures outside of having the discs as it would be less limiting to the disc nature of the game, but then there's not much desire to hunt for the discs themselves. unless they'd be more common than the mounts which is totally possible.
there's my analysis