9th grade physics student: Mr. Lipshutz, graphing is pointless and we are never going to need this.
Mr. Lipshutz: You're gonna need it on the exam, so shut up and start studying.
The student who whines about "when are we gonna need this" is really saying "I need an excuse for why I won't learn what you ask me to learn. Here's one excuse that has consistently gotten me sympathy from my parents and teachers outside the science department: If I fail, it's not because I'm a lazy bum, it's because the material isn't useful to me right this instant."
Never mind that American history has no immediate use outside the classroom, never mind that conjugations of être are useless except in the unlikely event I go to Paris, never mind that I'm never going to make a dime just because I understand Hamlet's motivations. I'm going to complain that physics is useless because some adult somewhere in my life (and most of my friends) will validate my intransigence by saying, "That's okay, honey, no one is really expected to understand that stuff, you're right, you're never going to need it. Everyone gets a D in that class."
When are we gonna use this, you ask? Right now, on this pop quiz.