The Golden Net directly deals with migration, trade, and the evolution and dissemination of culture.
Even if the language is not directly featured 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. 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 two other reasons why it is important to the story from a stylistic and narrative reason, though.
First is the opportunity to use Hharmari and its writing system in a visual adaptation. I love making comics, and most of the ones I make are wordless. Events and emotions are communicated though sequential images only. So, it would be an interesting next step as a cartoonist to make a comic adaptation of The Golden Net where this unknown language is used selectively for effect and immersion. In the same way that compound words are gradually introduced in the story text as the main characters grow older, a reader could pick up Hharmari symbols and words in a comic, not necessarily through direct instruction, but by repeatedly using key terms along with associated visuals. What happens then when a key term is combined with another term, or the individual parts of a term or compound-term are revealed to have a strange connection to other words? I'm not sure what feeling would come out of this, but I think it would be a unique one, equal yet distinct to the bizarre and wonderful things that can be done with images and languages we already know.
The second reason is to have a language that reflects and emphasizes the strange way that the world and characters are 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 language, we must break the language down into smaller parts, like its sounds, rhythmns 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.