"Neither of these examples is possible with f-strings. By providing a mechanism to intercept and transform interpolated values, template strings enable a wide range of string processing use cases."
As far as I can see, anything you do with the template, you could do before building the f-string or inline as in my intial example.
(As to your initial example: it's worth considering what will happen as you compose multiple bits of HTML via nesting to generate a large final page. The developer experience may become... unideal.)
* (barring undesirable hacks with inspect, etc.)
This gives you a convenient middle ground where you don’t need to learn a template library but still get safety. I can’t think of the code right now but I could see this being useful to pass in some dynamic HTML to, say, Django without having to remember to turn off escaping for that section. It can also be convenient for writing raw SQL without having to use prepared strings.