The decoding step (output of final layer -> word) is not strictly needed. You can feed the output directly into the next layer (Chain of Continuous Thought). You can 'decode' the output into things other than words.
How does that give the llm a way to speak in meaning that humans can’t understand?
You input text into the system, that moves through layers as chains of weights. Sure it doesn’t ever have to be output as text, but for the text-in-text-out machine to have any impact on the world around it, text must be generated.