There are a lot of things you can do with shadow DOM that you can’t with iframes or scope/donut selectors:
- transclude other elements (technically you can with iframes, but hopefully it’s clear why making something like this breadcrumb component would be extremely awkward [1])
- control and react to the display of child elements via <slot>s
- call methods on custom element objects
etc etc. It’s fine if it doesn’t solve any problems you personally have, but that doesn’t mean it’s not useful.