I studied few functional languages, mostly for academical purposes. Nevertheless, when I have to project a client-server application I always start adopting a Domain Driven
Have not tried it yet, but one place where I sure would like to use functional language, even though this might not be a reason to use it, is so that I can avoid what has been describe here
http://steve-yegge.blogspot.com/2006/03/execution-in-kingdom-of-nouns.html
I would like to just name this as a verb, and execute it
calculateSomething() instead of SomtingCalculator.Execute()