In my openwrt wan_6 interface, I see IPv6 and IPv6-PD as below,
So far, for self hosting services, I have been using IPv6-PD.
I also noticed all my LAN devices get IPv6-PD::SUFFIX
Just wondering what is IPv6 and what is the use of that?
The router needs, or at least should have, an GUA address. So the router can init connections on its own.
This normally should come from a dedicated prefix. It's part of the ISP network.
But the PD is an object and a route assigned to your network. The router handles them to number the downstream interfaces.
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With IPv6, in standard setups there is no NAT. Your router establishes a connection to ISP's router on the other end, configures IPv6 address for itself on that link and configures the IPv6 gateway and DNS using information from Router Advertisement from the ISP and optionally DHCPv6.
But in standard setup (without hacks like ndproxy) this one or two adresses are not usable for connecting devices behind your router - the ISP has to provide (delegate) you a prefix that you can divide into networks and use with your devices.
Not that I haven't annoyed everyone already with it...., but 
With IPv6 you can even build a (routed) infrastructure without any assigned interface addresses. Because, with IPv6, each interface needs/gets/has a Link Local Address by definition, this address is enough to build a complete routing path. Even my router has various GUA, it is the LLA which is used.
$ ip -6 route show default
default via fe80::1ff:fe01:40 dev eno1 proto ra metric 100 pref medium
But normally this isn't done, because one looses the ability to run any meaning full traceroutes. That's why routers (datacenter routers, not home user equipment everythingandnothing devices ) excusably have their primary address on loopback. But if an operator wants to see over which interface traffic is routed, then each interface gets its dedicated interface GUA.
If you somehow want to learn more, I can still highly recommend: https://www.oreilly.com/library/view/ipv6-address-planning/9781491908211/
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