Because "it" in these sentences is the personal pronoun. In English in most of the cases you need the personal pronoun.

Hi Serhat. In this sentence, they are describing future benefits of the action (to be more useful) to the same subject, in addition to the initial benefit (to be smarter). Like a flow-on effect. If the conversation is only speaking about the one subject, repeating a personal pronoun is not always required as it is implied by only having the one subject. The word 'thus' helps indicate that the future benefit is a result of the first one.

Here, "thus" is a connecting word. It isn't introducing a new clause. To see this, let's remove it from the sentence: "They want to make computers smarter and more useful." So "smarter" and "more useful" both describe "computers." Now, add "thus" back into the sentence. "They want to make computers smarter and thus more useful" means that their computers will be smarter and more useful, and that they will be "more useful" _because_ they will be "smarter."