Scribes are magic users that don't use mana! They are very different than a traditional spellcaster, because they invoke magical scrolls to cast spells. The downside of scrolls is the fact that they are destroyed when invoked (matter of the scroll converted to mana for the spell). But, as long as a Scribe has some stocked, they don't have to worry about lack of mana like other spellcasters.
They also have the ability to teach other magic users spells by creating instructional documentation. They can only teach that which they know, but they can learn spells from many of the different magic realms (Wizardry, Psi, Geomancy, etc.). From that perspective, Scribes can cast a larger variety of spells than most casters.
Scribes will also create random scrolls over time, which is always a great thing to see when you wake up in the morning. The Sage vocation can invoke the magical scrolls created by Scribes as well!
Finally, Scribes have the ability to learn ancient forgotten spells that no other caster knows, via the Runology skill. Of course, that requires finding some ancient runes...