NDP
IPv6 is not only about knowing your own address. A host also needs to know who the default router is, where local neighbors are, whether an address conflicts with someone else, and which prefixes can be treated as directly reachable on the local link. If you still split those jobs the way IPv4 often did, boundaries keep getting loose.
NDP handles that problem. It is not as simple as “turn ARP into IPv6 length addresses”. It brings neighbor resolution, router discovery, prefix advertisement, and address conflict detection into one ICMPv6 control-message framework for the IPv6 local link.