Raids are usually organized by Raiders, sometimes by Spartans. They normally choose targets that are away from any settlements, preferably defenseless peasants. Raid parties rarely have good weapons, but they make up for this with numbers and tenacity. Killing raiders isn't enormously profitable, but everybody has to start somewhere, right? At least the chance of Raiders mounting a Crackdown mission in response is very low; it can only happen if they happen to have very powerful friends.