The term "octet" is used in IETF documentation (for IP addresses, for example) to be specific that the byte is 8 bits in length. Historically the size of a "byte" on a system was machine-dependent. The industry coalesced around the 8-bit byte, and differentiated it from "machine word" in the 70s and 80s.
Like the sibling says, octet is useful when in a networking context, because bytes weren't uniformly sized, but also because communications protocols were sometimes only 7-bit.
Serial ports and modems often operated in that mode, and UUCP influenced mail and newsgroups to only use 7-bit data; requiring encoding for data with the high bit set. Protocols that specify octets are dealing with 8-bit bytes and don't have to deal with that.
