SAM's are the only defence against Spores, giant floating rocks that hurl towards your network. When fired, the SAM will take up half of its ammo, any missiles fired at a destroyed spore will automaticly home on another one.
These usefull missiles will have to be relocated or moved when your network grows! building more will just waste the ones already built. Amazingly, Blasters can take out Spores as soon as they hit the ground.