Where you start a new document you get a bunch of unmapped tiles. This is great for drawing free-form.
As an example of the problem: if you use "reidentify tiles", these are all assigned to tile 0 which forces you to use the Tile Placer tool to unmap the tiles one by one. (Can't even hold down and unmap many tiles in a line as you can while placing tiles, or unmap a rectangular area, or anything like that.)
Another example is when you have a bunch of tiles already placed and you just want to unmap them to modify them at once. If you have space on the current layer with already unmapped tiles, you could copy/paste the graphics from the mapped tiles into an unmapped space and draw there, but if everything is mapped you must create a new layer which is not always what you want.
As an alternative, why not have an option that marks every tile in the map as "untouched". Then when you draw, every "untouched" tile that you paint on becomes "touched", and a new tile is created and mapped to it. This would accelerate tileset creation and variation a lot, especially if entering this mode could be mapped to a key.