I'm looking at applications regarding conduit that will be beneath a slab. I understand that sheathed wire is not approved by code for inside conduit that is buried. I'm sure there's a reason why, but I'd sure like to understand this reason.

I understand that sheathed wire is not approved by code for inside conduit that is buried. I'm sure there's a reason why, but I'd sure like to understand this reason.This is certainly not true. While it is poor practice to use cable in conduit, it is not expressly forbidden.

You need to use W (wet) rated conductors in an underground conduit. Most THHN today is dual rated THHN/THWN.

One reason I like Conduit and individual wires is if wiring gets damaged, it can be replaced and you won't be screwed. If wiring was going to an island or adding a wire for switchleg, anything, It can save you/ anyone Money down the road. I don't even like romex in the walls, for the fact it is usually obsolete the moment it is installed. Things usually have to be destroyed for future needs.