Every sci-fi hero (or crew) needs a no-named 'red shirt' around at all times. Like a matador with his infamous red cape, the hero uses Crewman #6 as a proverbial magnet for any damage (be it collateral or fatal). If someone is going to get split in half by a three-headed jabberwocky, it's not going to be any of the actors (Oops sorry! I mean real spaceship crew members!) taking home six figures.