Languages contain a wealth of scientific knowledge. Indigenous peoples, whose languages are often endangered, know about their surroundings better than outsiders. Speakers of minority languages may have centuries’ worth of information on local geography, information about animals and the environment, navigation, botany, meteorology, and more.
If the last speaker of such an endangered language dies, then the vocabularies and science about indigenous peoples' environments are lost as well.
Languages contain historical value. If the language has been around for centuries and does not have a written form, losing the language could mean losing centuries’ worth of history and knowledge.
Like knowledge about the natural world, historical knowledge is important to preserve. Future generations of a people group can have access to their cultural heritage, and future researchers can trace human histories of migration and change.
Languages are scientifically interesting. Each language has a unique grammar system, and each language's grammatical and phonetic system are worth studying to better learn about the human capacity for communication. Studying language systems helps linguists address humans' brain development, cognitive capacities, and even communicative disorders.
Endangered languages are part of humanity's capacity for knowledge, and such knowledge should be preserved so we do not biasedly rely on the knowledge and culture of the few majority languages widely spoken around the world.