After 16 years of teaching, I know this question comes up at various times throughout EVERY year. I gauge the intention of the asker. Do they just want to waste time, so they can sit and zone out? Are they being ornery? Do they really want to know? I also have changed every year I teach, and as I ponder the question myself throughout the years.
The subject came up again during the last days of school when my seniors in calculus were going over good and bad things throughout their HS lives. One student mentioned the fact that she learned things she would never use again, and was maybe wondering what the point of it all was. My response to her was many fold but brief since others wanted to have their say on their HS experiences. I responded that even though I haven't directly used biology, for example, EVER after HS, I'm glad I learned it because now when news stories come up or research advances, I can be part of the conversation and have a semi-inkling of what's being discussed and what's at stake, and more importantly, I know enough to go find more information if I so choose. I also mentioned that learning a wide variety of things makes you a well-rounded person and it would be a shame if everyone only knew limited things because then their lives would be more constrained.
But in math class when students ask (and maybe I'm naive, but I think they truly want to know most of the time), I start out with, "you may never directly use this, but here is how it is used by others ....." and I make sure to have an example, or mention that I'll find one and get back to them.
I think a broader answer could be, "future employers want to know that you can learn challenging things that you're maybe not so interested in and then be able to apply them successfully in your work situation." Another answer could be, " learning and mastering and struggling through difficult-for-you concepts makes you smarter and more confident in learning in general." I guess we could also add, that when you stop learning or trying to learn things, then your brain starts to get flabby and liquidy and ooze out of your ears. That's not going to be good for anyone.