Subjobs rock, and if you're playing Final Fantasy XI you simply gotta have one. Properly leveled subjobs give subtantial statistical boosts to your main job, as well as access to abilities the main job could never use. In essence, they allow you to play two classes at once. The only restrictions are that the subjob may only be at half the level of your main profession, and that the subjob can only gain experience when it's swapped in and played as your main job.