Because every interface on your system, physical or virtual, has a link local IPv6 address your system needs a little guidance as to which one to use when sending packets to link-local IPv6 addresses. From an IPv4 perspective this type of addressing is an alien concept. But to IPv6, it’s everyday life.
…about the IP address of your default router (default gateway in IPv4-speak).
It’s tough to argue against the fact that most IPv6 addresses are not much fun to type. Being four times longer than IPv4 addresses and expressed in hexadecimal means things can get ugly on the keyboard pretty quickly. For people in the IT field one very common mechanism for testing IP connectivity is to ping the default gateway. And in IPv4 networks, the default gateway is always different for every layer-3 network. It has now been a thousand bajillion times in my career when I have either asked someone or told someone what the default gateway is for a host who is having connectivity problems. In IPv6 the ability (or inability) to ping the default router is just as helpful as it ever was in IPv4. But there are a few apparent problems/challenges: