The job becomes available at level 30. There are job specific skills added from that point onward. Failure to get those job skills will significantly limit your ability to play the role effectively.
E.g. White Mage skills
Regen, Holy, Benediction, Asylum, Assize and Tetragrammaton are all powerful heal spells which are just not available if you do not take up the Job and just stick with the class.