Even though we cannot read Harappan script, why is the knowledge that the Harappans wrote in script important?
In thinking about this question, I think you should not focus on the fact that the Harappans wrote in script but rather on the fact that they wrote at all. In other words, it would not make much difference if they wrote in script or in hieroglyphics so long as they had a written language. Your text discusses the importance of this fact at the end of the excerpt that you have included with your question.
Your text says that the fact that the Harappans wrote in script is partly important because writing is an “enormous intellectual advance.” People have to be able to think of representing sounds with symbols in order to create a system of writing. By this logic, the presence of writing would indicate that the Harappans were intellectually advanced. I would not use this argument myself, though, because the Harappans are not believed to have invented the idea of writing from scratch. Instead, they are believed to have borrowed the idea from neighbors, with the ultimate source of the idea having been the Sumerian cuneiform. Therefore, it is not (in my view) evidence that the Harappans were particularly intellectually advanced.
In my view, the more important thing about this script is that it shows how advanced and complex Harappan society was. Early societies that are not very advanced have no need for writing. Writing only arises when a society reaches a certain level of economic and political complexity. Such a society needs writing so that its merchants can keep track of their business dealings. It needs writing so that government officials can keep records of taxes paid and owed. Even if the Harappans did not invent writing on their own, they had to devise a writing system that fit their language and they had to go to the trouble of maintaining a group of people (scribes, for instance) who knew how to read and write. They would not have done these things if their society had not been complex enough to need writing. Thus, the most important implication of Harappan script is that it lets us be absolutely certain that this civilization was economically and politically complex.