Using a full Turn to cast a spell is usually for things like opening a portal or becoming invisible when you are not in combat. Outside of combat, casters can take their sweet time to cast a spell, so they don't have to make a WIL roll. This spell will take effect directly after that Turn so casters can't cook up a spell beforehand to cast during combat. This rule is there for when a caster wants to cast a spell outside of combat.
Combat in DURF should be a bit scary and stressful, so casting a spell during a fight it is a little risky.