The player holds a bunch of them and throws them on the floor to make a small trap. This essentially acts similarly to barbed wire, where if an enemy player walks on them, they will be damaged slightly, and continuously if they stay on them. Damage from Caltrops does stack, meaning that if one places more than one set of Caltrops in the same spot, an enemy will take damage from all sets simultaneously.
Caltrops are useful to a sniper to cover his rear or the entrance to the building/ tower he/she is in. They are small and easily overlooked, especially when placed in tall grass and bushes. They are also useful as a warning system to the player, since he will get a hitmarker on his crosshair for every enemy that walks over the caltrops. Another benefit is that caltrops don't disappear after being stepped on and remain in position for as long as the player is alive.