I commonly implement things in an imperative style as a quick hack, then if it gets use I translate it into a more functional style. It kind of just happens as I clean it up and refactor during revisits.

It might be a matter of taste, but I enjoy code built with functional abstractions that allow neat composable data flows and some caches loitering around. I find it also helps when adding UI. Sometimes performance could be better with mutation, but when I'm at that point I've already spent much more time tuning the thing with caches.