Design works as a language and to answer this question you need to know the fundamentals of language itself.
In every language, every used element has meaning, and the structures and ways we combine these elements expand upon their meanings or create new ones altogether. Clear and consistent rules are what make languages successful. Consistency makes every language easy to learn, teach, and understand.
Pretty clear that to call a set of designs a DESIGN LANGUAGE, we need to have a formal, unified system of meanings to communicate with our users intelligibly. Colors are a great example of semantics representing predefined messages in the language.
Design language ensures that all the teams throughout a company can follow the same rules and methodologies. This allows them to have a set of reference points and coordinates that makes the design process efficient and free of confusion.