The mnemonic acts as a backup mechanism for a set of keys. Entropy is encoded in human-readable form using words from a fixed vocabulary which are combined into a mnemonic. This mnemonic can then be easily backed up by a person via whatever means are appropriate for the security level.
BIP39 is a widely adopted standard for such a purpose with robust implementations in multiple programming languages supporting vocabularies in several languages. BIP39 is therefore adopted as the standard for generating a mnemonic and converting the mnemonic to a seed which forms the basis for the rest of this specification.