> Is IPv6 really that widely used?
Mobile carriers use it almost exclusively, which is already a huge chunk of the internet, and newer ISPs are switching to it too.
> I'm supporting both because I heard it's good to support both, but I'm not sure what the actual benefit is.
The benefit is that you allow IPv4-only and IPv6-only clients to connect.
I accidentally became the user of an IPv6-only device a while back for some obscure reason I never could figure out. Let me tell you: There are no IPv6-only users. Absolutely nothing except Google, Facebook, and YouTube works. Any website not in the top 20 are IPv4-only. It was so bad I briefly thought I didn't have an internet connection at all. Anyone stuck on an IPv6-only connection would immediately cancel their contract on the grounds that they don't have de-facto internet access.