Let's consider a simple use case.
You want to set up a router that is your IPv6 upstream in an existing IPv4 network. You would like it to work as an IPv4 DHCP client, too - not fight over dominance with your existing router.
That's what one sees when creating a new interface. Is this use case a DHCP client or a static address?
The default configuration suggests a solution - there can be two interfaces, lan and lanv6. Still, it's not very obvious.
Should "Ignore interface" be clicked? If it is unchecked, will packets be broadcasted using both stacks or only IPv4? "Start" and "Limit" from the same tab look like they are for IPv4 only - and the tab has no IPv6 exclusive configuration.
There are more like that. You can try setting a device like this up without losing track of what services are responsible for what as an exercise for yourself. The flow for configuring DHCP over different stacks is severely underdocumented, and actual answers are often counterintuitive - I could not manage to do this without enabling a DHCP server on both LAN and LANv6 and creating some arcane configuration.