Throughout the course of Final Fantasy Tactics you'll end up recruiting a lot of different characters to your team. You start with 6 extra dudes, more join from different plot points, and you can recruit extras for cash if you want. For the most part you get to use 3 or 4 of them in a fight which means you have a lot of people on your team just sitting around. Like poor Kimahri, they need experience but you can't get them into your fights. What are you supposed to do with a small army of people who can never get into fights?
Well, first off if you're in the habit of letting people die then you need some replacements on the bench. I am not in that habit. I make sure that most of my team has access to some sort of resurrection spell/ability/item in order to keep that from happening and I'm more than willing to reload if someone dies. So, I don't need extra for that.
Next, theoretically you could want different characters specialized in different roles and sub different ones in depending on the fight. But since you can only level the ones you're using at any given time and since the game uses a job system you can just change your current team into a new team if you really need to bring a different formation into a battle. So, no actual need for extra here, either.
The actual use for them unlocked for me today when I hit chapter 2. You can send extra guys off on errands in each town. The exact details are unclear but it seems you pick an errand, pay some cash, send some guys off for a predetermined amount of time, and wait. You can't use them for anything else while they're gone and they might bring back some sort of reward when they return. I know this system was used again in Final Fantasy Tactics Advance 2 and I seem to recall it let you know who might be good at a given errand. This time there seems to be no indication of potential rewards or chances of success. This makes me sad, so I decided to turn to the internet for answers.
The first thing I discovered put my mind at ease. While you can fail these errands there is no permanent consequences for doing so. You still get a small reward for trying and the errand respawns again in a month. Some errands are only available in a given month but if you wait an entire year you can try again. (Game years, where every movement on the map takes a day, not real years.) So even if I screw things up and send the wrong people off it just costs me money, not access to rewards.
Determining success is actually pretty easy, though only if you have a guide on hand. Basically the job, level, and bravery/faith levels of the people you send help determine if you succeed or not. Rewards are always JP and gold, and may also give some random tokens with story background information. So skipping these seems like a fine thing to do, but since I have extra guys just lying around I might as well send them off for more gold. You can send 1-3 people but it seems strictly superior to send 3 since the contributions of each character add together. I only sent 2 on my first one so I bet it fails. Oh well!
I've been really appreciating the new translation on the PSP so maybe I'll try to get all the wonders and artifacts from these errands so I can read more background plot. It can't hurt to try, anyway, since I wasn't going to use these dudes for anything else!