We're running out of IPv4 addresses (the stuff that computers communicate with), but IPv6 will solve that problem. Some ISPs have already adopted it (which is excellent), but others have mentioned they have absolutely no plans of supporting it.
You may be interested in these active petitions
By having a requirement for public ISPs to offer a free IPv6 address, more consumers of the web will be able to access content using it, and therefore moving us closer to a 100% IPv6 world. It's sort of like moving to green sustainable electricity, except that it's a lot easier to do, as these ISPs are able to buy huge amounts of IPv6 space for extremely cheap.
There's no downside (that I'm aware of) for customers having IPv6, and if anything, it will help promote for more websites to upgrade.