They are designed to move long lengths of steel, lumber or what have you more easily without having large doorways and in and around a building. Using a regular forklift is much more difficult to move material of this nature through a doorway and it has tendency to start rocking on the forks when you try to move around the load and usually tips and slides off sideway from the forks.
You can pull up next to a flat bed trailer, traverse the mast, pick up the load, retract the mast back and set the load on the deck. It is a much "safer" way of handling long items. They work very well moving flat bar metals that bow when you unload.