There are more and more networks that are native IPv6. And IPv4 connectivity is done with double NAT, NAT64, 464XLAT and other heroics.

The faster we can allow native IPv6 end-to-end, the faster we can get rid of the complex IPv4 compatibility technologies and simplify the entire internet. At some point, you may start to see networks with no v4 connectivity whatsoever (maybe they already exist, but I can't imagine them being useful), so this allows those users to reach github pages.

To directly answer your question, it's investment and maintenance in infrastructure, not for user features.

> maybe they already exist, but I can't imagine them being useful

They are rare but do exist! Here's a couple of reports from the IPv6 subreddit: [1] [2].

[1]: https://teddit.net/r/ipv6/comments/he38ga/work_has_turned_of...

[2]: https://teddit.net/r/ipv6/comments/n5y5oo/update_on_not_bein...

It's nothing critical but some ISPs use multiple layers of hacks to work IPv4 and certain types of networking become very unreliable over it. Things like long running connections will be dropped because the ISP routers do not maintain the NAT mapping forever.

For github pages its probably not going to make a huge difference but for things like SSH or file transfers it could be an issue. For things like video game servers or home servers you just flat out can not work them on v4 if your ISP uses CG-NAT.