What do you people DO that you are doing tasks like this? I've always loved FP from college, but never ran into problems like this that would actually warrant using one
In a larger sense, and probably more relevant to your question, it's not about any particular application space where FP is "warranted", but rather about being familiar enough with enough different languages and architectures that I can look at a problem and see a variety of ways to solve it. Second, I always build a prototype in a "weird" language that I don't intend to put into production, because the prototype is more there to understand the problem than to come up with a production-ready solution. Weird languages add friction to deciding to productize the prototype, and therefore encourage me to really consider what tech stack is appropriate for the actual product.
Finally, it's probably worth noting that I rarely work on anything particularly interactive. If you're not touching GUIs or web stuff, you'll often find that you're a lot less constrained on your tech choices, because you don't actually need all that much from libraries.
In such languages, you want to be able to use the entire language at any arbitrary point, including at a point where execution has halted for the moment because of some error that has occurred in some arbitrary place. The condition system provides a nice set of tools for doing that.