In Spanish, you add an untranslatable a before a personal direct object! That means, before PEOPLE you say a! Isn't that strange? To say vi una persona is a grammatical mistake, since una persona (a person) is a human being. You have to say vi a una persona.
So, even though we don't translate את into English, it still bears an important function in marking the direct object when it is definite. It is a grammatical particle, like the Spanish a personal. Grammatical markers are as important as anything else, as in Greek we have case endings to mark noun functions. We might term those meaningless (especially the accusative) in terms of English, but that doesn't make them meaningless to Greek!