The targeting for updates. Something like hotwire/turbo frames allows you to push a form's response to any turbo-frame. You can also push updates to multiple frames at once. They're matched by frame ID, much like target="", but in the case of iframes, the iframe is the only thing that can update the interior HTML. (other than just changing the URL from the parent application)

iframes are also in a rather locked-down security model, even if they exist on the same domain. (Requires message passing, strict CORS rules, strict cookies rules etc)