The Mechanics of Spell Casting

[Nahara]

The conjurer quest line explains it pretty well. I haven't even got a level 1 BLM or ARC to say whether theirs has any but basically the conjurer calls upon nature's energy and redirects it. There is a girl who refuses to do this but can still cast spell using her own life force. Bad things happen I don't want to spoil it but conjurers merely need the will to use but use the energy from nature basically. Much like cars. Don't run cars on lifeforce.

There's also the fact that Conjurers have to be very careful with the magic that they wield. Unlike the other two forms of magic, conjury draws it's power directly from nature, and it was essentially the reckless misuse of this fact that lead to an umbral era. Using this magic too much essentially began to bleed the planet dry of it's natural life-force. Think of it like what happened to the world in FFVII when Shinra started turning Mako, the lifeblood of the planet, into electricity ("A Mako reactor outside of Midgar usually means there's nothing else out there").