Often when you picked up a mission from one of the terminals, there would be a logical reason for the NPC who wrote the "help wanted" message hiring you. A farmer might want you to kill a swoop gang that was terrorizing him, or a cowardly Trandoshan whose reputation depended on him killing an animal he was too scared to face might want you to do the job for him and then let him take the credit. This wasn't always the case, however; to paraphrase from memory: "So I was walking along one day, and suddenly I saw this THING, and it was UGLY! It's so ugly that I want to hire somebody to kill it! Blech!"