What problem is this trying to solve and does it actually succeed at solving it? I‘m struggling to see the appeal given that the JS still needs to model the internal structure of the template in order to fill the slots.

The Shadow DOM can auto-fill the slots in the case where a web component has slot fillers in the main DOM. You still need JS to invoke/create the shadow DOM, but in that case your JS might be minimal and not need to model the interior structure.

But the big problem that template tries to solve is building DOM fragments that are parsed but not "live" in the open documment. Before the template tag there was no good way to do that other than JS and createElement/createElementNS, and that has always been slower than the Browser's well optimized HTML parser.

Also, the slot tag does solve a minor problem of being the first tag whose out-of-the-box (browser CSS) behavior is `display: contents;`. It's obviously not a huge lift from the CSS one-liner, but there are still some uses for it even outside of templates.