Every once in a while, the question of the usefulness of knowledge rears its ugly head. Usually in the context of education, where it is accompanying the question of what to teach the kids (and where to allocate limited monetary resources). At other times, as a connotation to the accusation that you are wasting your life by not learning something else - such as the art of accounting or lawyering.
The worst part about it - aside from the "can we get away with not spending money on this" and "why are you wasting your life" parts - is that the measure of what is useful always is a retroactive quality. You never know what will be useful to know until you are in the situation wherein it would be useful, and by then it is usually too late to learn fast enough to make a difference. The only way to be prepared is to have learnt these things ahead of time. You never know what will be useful until it is.
Moreover, the notion of useful knowledge is usually defined with a few specific situations in mind, ruling out most of the vastness of human experience as irrelevant. This is seen most clearly when it comes to educational regimes focused on preparing kids for work, to the exclusion of everything else. Everything is geared towards this one particular purpose, and those things not specifically geared towards this purpose are ignored.
What is the workplace utility of critical literacy, of knowing how to be a supporting family member, or of fluency in the arts?
What is the workplace utility of having read a poem that makes the experience of having a bad breakup more bearable?
When push comes to shove, the notion of useful knowledge always depends on just exactly who it is supposed to be useful for. More often than not, it tends to be someone who is not you. Their exact identity is shared on a need-to-know basis, and you do not need to know.
The question of whether knowledge is useful or not is never asked without hostile intent. Let there be no mistake about that.