According to Jehovah’s Witnesses why did John write the words found in John 1:1?

Jehovah's Witnesses translate John 1:1 as: " In the beginning was the Word, and the Word was with God, and the Word was a god." whereas its more generally translated as something like : "In the beginning was the Word, and the Word was with God, and the Word was God." i.e. the NWT adds the word "a". This is justified because the Greek language of the first century did not have an indefinite article , and they feel that this rendering of the verse makes it more consistent with the rest of the bible (which they see as teaching that Christ is an Archangel, rather than a person within the Godhead). If this is the case, why do they think John used this phrase? Is that what he was trying to communicate? If John is simply trying to say that Christ was with God, and through him all things were created, this clause could be removed from the sentence- and the passage would become clearer, because John's Greek speaking audience wouldn't have to wonder if John means Jehovah or simply some lesser god. Additionally, this would avoid all the problems that they see as coming from the miss-translation of the verse being used to support trinitarianism EDIT: This Question is flagged as similar to another . The difference is subtle- that one asks what textual support is there for the verse, this asks, assuming that the JW understanding of the scriptures and Johns intentions are true, why John would write the verse as it is - appearing to endorse at least the divinity of Christ.