Basically what lore said. Plus you'll learn and be able to understand what works for not only the individual jobs, but for your party as well. For instance, BLM is always a good choice for balance, but if you have a move intensive fight with many AoE and your choixes are between a BLM and a DRG, giving the balance to the DRG is better since the buff will have more uo time over the BLM who cant attack and move as easily.