It's the epitome of applying the KISS principle for building interactive web applications, not in small part because it allows the developer to keep the UI code in a single place, backend templates, instead of spreading it around[2]. In my particular experience, I've found that htmx pairs really well with my backend microframework of choice, flask.
In fact, using htmx means that there isn't a division between backend and frontend as much as there's a backend with some extra attributes in some of the tags in the templates, and a few extra rules deciding which template to serve at specific endpoints.
[1]: https://htmx.org/
[2]: Having spent most of my professional life deep within js/ts frameworks, I thought that progressive enhancement was either a pipe dream or required duplicating at least a significant portion of the UI code on both ends. Htmx showed to me that such ain't necessarily the case; `hx-boost` in particular is a godsend.