When attached to a cart by the Cart Assembler, the Torch Placer will place Torches as needed to maintain a set light level, in order to provide light in dark environments. The three inventory slots in the Torch Placer's GUI should be stocked with Torches. The bar underneath the inventory in the GUI shows you two things: the current light level of the block the cart is on, indicated by the yellow bar, and the target light level for the Torch Placer, indicated by the red line. The default target light level is eight (the minimum level to prevent mobs from spawning). It can be used in tandem with Drills and a Railer while digging underground in order to prevent mobs from spawning in the newly dug tunnels.
Places torches near the Track as the cart passes by in order to maintain a specified light level.