What is the key from crypto_stream_keygen used for in libsodium.js?

[Jack-Edwards]

Libsodium.js symbols: https://github.com/jedisct1/libsodium.js/tree/master/wrapper/symbols

I noticed the ChaCha20 and XChaCha20 stream ciphers each have their own corresponding keygen functions in libsodium.js. These are crypto_stream_chacha20_keygen and crypto_stream_xchacha20_keygen.

What is the key from the crypto_stream_keygen function supposed to be used for? Can it be ignored for the purposes of building a library that wraps libsodium.js?