Sappers are a very useful unit to have. They can lay traps, and essentially decimate enemy forces just by using a bit of SP. Much like the Pyro Technicians, Sappers are able to set down traps that take time, and SP to build. once they are built, they will be triggered by an enemy stepping on the trapped area, causing unblockable damage. Sappers also have the ability to open Floodgates which are important to a few story missions.
Sappers can set traps, open dams, and dismantle enemy traps. Sappers can be substituted for Infantry but they do not carry shields for defense.