Where would Robin Hood be without his Merry Men? King Arthur without Lancelot and the Knights of the Round Table? The Lone Ranger without Tonto? Sherlock Holmes without Dr Watson or Peter Pan without the Lost Boys? Even the most heroic figure sometimes needs someone to watch his back - a confidant, a pal, a lackey, or just someone to polish his boots and care for his horse. The knight who single-handedly slays the dragon is seldom really alone.
PCs are no different. A charismatic military leader needs subordinates. A well-to-do gentleman may need a valet. Everyone needs to obtain basic services from somewhere, and few PCs can do everything themselves. Enter the Cohorts and Hirelings of the Shining Jewel. They may be motivated by money, a desire for adventure or genuine loyalty to a PC or cause.