[fanf2]

A scoped IPv6 address contains a network interface name, which is used to disambiguate which network a link-local address belongs to.

It’s relatively common to encounter them in some network configurations where the routers use link-local addresses rather than global addresses in their RA packets. They are also used in SLAAC, because a device can configure a link-local address without knowing anything about its environment.

[ I think in the past there might have been an idea to use them to disambiguate site-local IPv6 addresses, but network interfaces are not sufficient in that case, and site-local addresses were problematic in lots of other ways, so they have been dropped from the specifications. ]