I've recently learned that 者 is used to nominalize an action, whereas I used to think its purpose was just to assign an action to an actor and reference the actor. But I'm going through Fuller's book, and he uses the structure in the title. For example, he says that instead of "从X坠", we can write "X者，所从坠也。" This emphasizes X and makes it the topic of the sentence. Another similar example is "剑者，涉江者所求也," where "剑" is emphasized, and that is the object which the "涉江"-er seeks.
I don't understand why we need 者 in these cases. 剑 and any X are already nouns, so why do they need nominalization? What is the 者 doing?