How to answer "functional programming is useless"?
What can I use to show the usefulness of functional programming?


This is the wrong question, and by trying to answer it you're falling into a trap of accepting and reinforcing the students' misunderstanding of what university is. If I had applied that standard of value to the courses in the bachelor's degree I studied, I think I would only have attended 30 hours of lectures over the three years.

You may be right that most of them will not end up earning their living by programming in functional languages, although given current industry trends I wouldn't be entirely surprised if you turn out to be wrong in the long term. But if their goal is merely to gain a superficial knowledge of whatever is currently popular in industry, they're not just in the wrong course: they're in the wrong institution, and should drop out and find a bootcamp.