According to Link-local address on Wikipedia:

In IPv4, link-local addresses are normally only used when no external, stateful mechanism of address configuration exists, such as the Dynamic Host Configuration Protocol (DHCP), or when another primary configuration method has failed. In IPv6, link-local addresses are always assigned, along with addresses of other scopes, and are required for the internal functioning of various protocol components.

The citation refers to RFC 4291, but that simply asserts that "all interfaces are required to have at least one link-local unicast address" and does not identify the other relevant "protocol components." Why does IPv6 require link-local addresses, even when DHCPv6 or some other method of assigning a routable IPv6 address is available?

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    Because they are required for the internal functioning of various protocol components. No one RFC lists all of those various components. It's a fundamental part of the protocol, so it's always available for anything one can dream up. One simple case: routing is always done to the link-local address of the RA origin. (static routes are up to the user.) – Ricky Beam Jun 28 at 12:37
  • DHCPv6 is very different than DHCPv4: "The availability of these features means that a client can use its link-local address and a well-known multicast address to discover and communicate with DHCP servers or relay agents on its link." IPv6 Link-Local addressing is used for many things, and the question of , "Why?" needs to be asked of the IETF (you can find the authors on the RFCs). Anyone here answering will just be guessing and speculating, which is off-topic here. – Ron Maupin Jun 28 at 16:35

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