Link-local addresses (LLAs) are needed in IPv6 because IPv6 doesn't have broadcast. IPv6 uses multicast instead.

Broadcasts go to all IPv4 addresses in the subnet, multicasts only go to those who subscribed to a multicast group. To subscribe to a IPv6 multicast group you need an IPv6 address. So all IPv6 interfaces will have at least one LLA self-generated.

One thing that IPv6 uses multicast heavily for is NDP, which is the IPv6 version of ARP. This is how IP addresses on your LAN/WLAN are converted to MAC addresses which is required info for the NIC in your node to talk to another node on your Ethernet LAN/WLAN.

End users don't typically have to use LLAs directly but you can use them if you want to 100% ensure things won't leave your LAN as routers don't forward LLAs.