Is it required to mention boyun in this sentence:

Bir gün boyun uzayacak.

The verb uzamak already conveys the meaning of growing taller (which obviously refers to a person´s height). Again this is one of many examples where one says the same thing twice.

No. It is obvious that "tall" refers to a person/object having a certain height in this context.

What would be the answer to your question (what)?

"It" refers to a specific thing or animal in this case, so we both know what we´re talking about and "it" always refers to a person or an animal (although I disagree with the latter). You point at a plant, for example, and ask me: will it grower taller? Of course it will (hopefully).

You seem to have difficulties understanding that not everyone values the same priorities as you obviously do. Apart from that: boyum refers to a person while "it"can never ever refer to a person (unless in a pejorative sense).