Because the people that came up with IPv6 didn't think to ask if they should, just did it because they could.
(My Bachelors is in Network Engineering and I held Cisco certs, I don't use IPv6 for home networking. Way to easy to expose your clients to the internet at large.)
But basically if the IPv6 starts with f then it's not going to leave your LAN without a router relaying it, similar to IPv4 NAT. https://ipv6-test.com will tell you if your client is using IPv6. Unless you know you have to have it then it's better to just turn it off completely. You don't need it for a LAN.