The primary reason that some homes need it is that moisture can come up through the concrete in some circumstances. I know that when I was reading ceramic tile installation books, they always mentioned taping a 12" square of clear plastic to the center of the floor with duct tape sealing all 4 edges. They said to leave it there for a few days and then peel it up. If there is a damp patch in that spot, they recommend installing some sort of subflooring before tiling. It has to do with the ground water pressure in your area. I'de think this would be more important with a carpet installation to prevent a musty smell.
When I go to HD or Lowes this weekend, I'll get the product name. I'm sure that we can find literature explaining it's useage on the net...
I just ran a Google search on tile +concrete +plastic +moisture.
Here is the very first article that I clicked on