If I’m focused on justifying what I’m learning by the necessity of my need to use it, then I’ll naturally seek to learn the things that I need in order to do the things that I want. We may argue that we do not seek to only teach our children that which they seem to need to know, but do we trust ourselves to stay the course?
Without realizing it, we, as parents, can easily fall into the existential worldview trap. We can claim that our child should learn philosophy, but we may live our life only seeking the knowledge and skills that we will use. If we are wondering secretly if our child needs to know how to apply the Pythagorean Theorem in order to be or do a certain job then we may be crippling the argument before we even get to it. How do we expect our children to do hard things now if we don’t model the importance of doing hard things for the sake of doing them? We seek God by reading and understanding scripture because He is worthy to be sought. We may have already read the entire bible, but we read it again.
It is inaccurate to bundle the study of mathematics into the bucket of knowledge as something to be learned and studied and dumped out onto the heap of knowledge. We won’t use the particulars of linear equations in life, and that’s okay. We study linear equations because by studying them we learn about relationships, truths in life, and God. We may use them in the future to compare two scenarios and make decisions based on this information, or we may not. So the answer to the simple question muttered in frustration "when will I use this?" may be: “you won’t,” and that’s okay too. It’s okay to force ourselves to do hard things that we may never see again because life isn’t about the destination, after all we’re all going to the same place. Rather it's about what we do with the time and resources that God gives us on the journey. How do we use the gifts he has given us? If our goal in education isn't bringing us closer to a profession to be mastered on earth, then to what is it bringing us closer? That is our true goal, end, focus, and motivation. The chief end of man is to glorify God and enjoy Him forever. We cannot glorify that which we don't seek, and we can't seek God in ignorance of the things that He has done, and we can't know the things that He has done without seeking knowledge. Education is just one tool on the path.