The Golden Net directly deals with migration, trade, and the evolution and dissemination of culture.
Even if the language is not extensively used in the story text, wrapping one's mind around how a language like Hharmari can develop (especially alongside the characters as they grow up, aquire various new languages, and see them shift over time) helps us feel the way that their world is in motion. Additionally, as the writer of the story, wrapping my mind around that concept has naturally made me curious about creating the language itself. Thus, it exists.
There are a few other reasons why it is important to the story from a stylistic and narrative reason, though.
One reason is to communicate feelings and information without being overly literal and direct. If the ideophones for waves crashing and receding on the beach (mi-ro mi-ro) and waves rocking things left and right on the surface of the water (mo-ri mo-ri) sound similar to eachother and the names of various characters, what impression could that have on the reader?
Another reason is to have a language that reflects and emphasizes the way that the world and characters were formed. Dead bodies floating in the sea are stitched together and reborn as something new; languages can be merged together to create something unknown yet familar.
Additionally, there is usually no sound in a comic or a written story. The narrator we have in our minds when reading a language we know may have a sort of non-voice, one that communicates textual and auditory information, but does not really have volume. When a barrier is put between us and the information a language is conveying, such as by using an invented script or language, we must break down what we see into smaller parts, like sounds, rhythms, and patterns, and then connect them to visual and emotional context clues. At that moment, our mind's narrator gains an active voice, and therefore, the story gains a new sensory dimension.