The article I linked goes into the details, but basically a relative approach constrains your markup: The positioned thing must be a child of the relatively positioned anchor or a wrapper element, and you often cannot sensibly position it without using JS to check the location of the anchor. E.g. a popover menu for a button must check if the button is close to an edge of the viewport and position the menu element appropriately. Anchor positioning will do this automatically.

Also, if support for multiple anchors is included, then it opens up some very interesting capabilities to do things like draw arbitrary diagram connectors between elements: https://kizu.dev/anchor-positioning-experiments/