You don’t need your spells written down to cast them, but it makes it easier to get it right. Grimoires are highly sought-after artifacts among sorcerers. If you cast a spell from a book or a scroll, its rank is considered one step lower than usual. In combat, you must spend a fast action readying your grimoire before you can cast the spell. Writing down a spell requires you to first cast it once (chance casting doesn’t count). Then, you must spend a Quarter Day with quill in hand and make a LORE roll. Writing down a ritual takes two Quarter Days.
So, from my understanding the only benefit would be you don't need an Ingredient to cast once the spell is written in the grimoire. However, it also seems the spell would be one Power Level lower than if it was cast normally?
So, what is the point in casting from the book?