Once it is understood that the word "other" is added five times by the NWT Committee to their rendering of Colossians 1:16-20, but NOT even once to John 1:1, that will give a fairer representation of their view that Jesus is an agent of creation. Also, to keep matters clear, it needs to be pointed out that they do believe Christ is deity in the sense of being "a god" (only one that must not be worshipped as the Father is worshipped).
The question is based upon why Christ is presented as the agent of creation, by Jehovah's Witnnesses. Again, for clarity, it should be pointed out that they speak of him as the MAIN agent of creation, but that the Father created Christ first, so that the Father is the initiating agent of creation, choosing to then use this one, whom they also call Michael the Archangel, as an instrumentality, yet without him deserving the title of "co-creator".
Only now that the question has been clarified can the Jehovah's Witness way of looking at it be fairly understood. It soon becomes obvious that their understanding is based upon their belief that the one spoken of in John 1:1-3 and Colossians 1:16-20 had a starting point in time, and was the first and only direct creation of the Father. This is proven with this quote from their literature, re. John 1:1:
"This one was in the beginning with God. Since Jehovah is eternal and
had no beginning, the Word's being with God 'from the beginning' must
here refer to the beginning of Jehovah's creative works... Col.
1:15... Thus the Scriptures identify the Word (Jesus in his prehuman
existence) as God's first creation, his firstborn Son...
The Son's share in the creative works, however, did not make him a
co-Creator with his Father. The power for creation came from God
through his holy spirit, or active force. And since Jehovah is the source of all life, all animate creation, visible and invisible, owes its life to him. Rather than a co-Creator, then, the Son was the agent or instrumentality through whom Jehovah, the Creator, worked." Insight on the Scriptures Vol.2 p.52, 1988
This clarifies another misunderstanding many people have about what Jehovah's Witnesses mean when they say that he is spoken of as "the agent of creation". Their meaning is that Jesus is simply an "instrumentality". Jehovah did all the work, simply using Christ (a bit like how a semi-conductor works? - that's not a JW phrase or idea; neither is the illustration of how a human can act as an instrumentality for a spirit messenger to speak and act; I'm just struggling to distinguish between an agent and an agency. I did look at Vol. 1 under 'Creation' p.527 but that offered no further insight as to the Son being an instrument through whom Jehovah created everything, indirectly.)
"Why is the word 'other' added?" The direct quotes given from their "Insight" book answer the question, from their point of view. It also helps explain why they insert the word 'other' into Colossians 1:16-20 no less than five times. The word is not in the Greek text, and if the translation stuck to "by means of him all things were created" etc. that would leave open the mainstream teaching that Christ was never created. Adding "other" detracts from that while bolstering their teaching that Christ was the first and only direct creation of Jehovah God.
"What are these 'other' things?" It must be repeated again that the NWT does not add 'other' anywhere in John 1:1-3. But it does in Colossians 1:16-20 and that has been answered: all things other than the Word of God himself. It does seem surprising, therefore, that adding 'other' to John 1:1-3 was refrained from. Had it done so, there would have been consistency.