Projectile modifier spells that add an arc of material between multiple projectiles. Arc spells require a minimum of two projectiles in order to function
Poison Arc is a modifier that causes poisonous particles to flow between fired projectiles, inflicting that status on any creature caught in the streams. Unless the spell naturally produces more than one projectile per cast, like Triplicate Bolt, it must be used with a multicast modifier to work.
If you can have poison fungal shifted to something useful (or harmful to enemies), this is a very good way to create much of it.
As poison is more dangerous to the player than the enemies, and no perk existing to block the effects, it is questionable whether there is any other effective way to use this modifier.