Why clients use a changing IP instead of a MAC address?

Trying to understand Clients in groups and why using IP addresses when they can be changed instead of using Mac addresses. Does this mean next time my device gets a new IP address I'll have to add it and to the group I want it in? Or I'll have to set the device with a static IP? (More difficult to do with ipv4). I'm trying to setup a group for my work devices as only they should be white listed for certain VPN services etc.
Pi-Hole 5 is running on a Raspbian GNU/Linux 10 (buster) with DHCP running on a Google Fiber ISP network.
Raspberry Pi 4 2gb

There is an already existing discussion on this topic, with an answer from a developer: