[laksjd]

NAT. If your server is only available via IPv4 then the client (user) needs an IPv4 address. Sadly there are so few IPv4 addresses that virtually everybody has to share with at least some people/devices and go through NAT (your home/office router, or for countries where ISPs don't have large ip blocks, some large NAT style router at your ISP). NAT is a horrible hack and causes all sorts of issues (port forwarding being the most common issue).

If you make your server available on IPv6, too, then I can use my non-shared IPv6 address to communicate with you which means that the packets don't need to undergo NAT which means a less horrible, faster connection without messy port forwarding requirements.