I think this htmx essay [1] addresses the tradeoffs you may be getting at if you're thinking "why html vs js+json": the gist is that html is self-describing to the browser, so the backend can make changes to the html it produces and the browser will understand it without the frontend code needing to be updated
If you're instead thinking more broadly in terms of "structure vs layout", I think the same reasoning for using something like tailwind css or higher-level web components may apply: i.e. the material you interact with is closer to your problem domain
htmx mixes logic with "templates" in the same manner that HTML mixes logic with templates: hypermedia control information is embedded in the hypermedia in the form of attributes
Maybe I should clarify the type of logic I mean. Standard HTML contains instructions for how content should be rendered. But it has no control flow structures to control what content should be rendered.
Embedding loops and if/else logic in htmx tags creates scenarios where the content is potentially modified in the rendering step, meaning you can't rely on just looking at the data the backend sent over the wire to determine the source of a result that renders incorrectly. Instead of having control flow in one place, and a single source of truth, it creates an additional point of failure.
Stock price showing up wrong? That might now be a problem in your backend logic or in your complex htmx engine, buried in some template tag.
You don't need to mix logic with templates. I just produce a context (result from database queries and some such) and pass it to a function that produces HTML from the data in the context. The data must not be changed in the templating function. This is something I try to avoid as well in order to maintain separation of concerns in the backend code.
Edit for clarification: The context holds the data that is rendered as-is by the templating function. So any transformations from the "raw" database data and the context data happens beforehand and the templating function just turns it into HTML.
