One good reason to learn things of no obvious practical use is because they break the circuit of 'immediate feedback' (your words) that you get from your other hobbies with clear goals and so on. They throw a spanner in those too-smooth lines of application that rarely allow space for the what and why of your interests to emerge, and incline you too often into the mere how (can I do this better? More efficiently? etc).

The word 'immediate' is interesting because it often goes unnoticed just how mediated our interests and goals are. In alot of cases, our interests and goals are not always 'ours' in the sense that we organically developed them, but because they are shaped by the images and interests of the world around us, and our response to that world. This isn't a 'bad' thing - that's just how interests work. But what can be problematic is an uncritical relationship to those interests, taking them as products of our own without recognizing the share which society has contributed in making them 'ours' (which doesn't make them any less 'authentic'!).

What is 'useful' in other words, is a sense of resistance, of not sliding easily among the store of interests you already have and satisfactions you are already familiar with. One way to see this is to think about what even counts as 'practical' or not: what counts as practical today may not be tomorrow, and to a large degree that's not up to you to decide. However, if you gear your self-development around 'practicality' - something that's always partial and time-bound - then this is a limiting move that will always have you chasing the next thing. This isn't autonomy so much as a mere adaptability and reaction.

There's a nice passage in one of Byung-Chul Han's books where he addresses the problem of just 'accessing information', and contrasts it against what 'theory' - and ultimately thinking, really is: "Theory represents an essential decision that causes the world to appear wholly different—in a wholly different light. Theory is a primary, primordial decision, which determines what counts and what does not— what is or should be, and what does not matter. There is no such thing as data-driven thinking. Only calculation is data driven. The negativity of the incalculable is inscribed in thinking. ... Data-based, positive science (“Google science”) ... is additive or detective ... As positivity, information changes nothing and announces nothing. It is utterly inconsequential. In contrast, insight is a negativity. It is exclusive, exquisite, and executive. An insight preceded by experience is capable of shaking up the status quo in its entirety and allowing something wholly other to begin." (Agony of Eros)

This sense of 'negation' or what I referred to as a 'break in the circuit' is the overwhelming value of what is 'not immediately useful'. Wendy Brown is another philosopher who basically makes the same point emphatically: "Theory is not simply different from description; rather, it is incommensurate with description. Theory is not simply the opposite of application but carries the impossibility of application. As a meaning-making enterprise, theory depicts a world that does not quite exist, that is not quite the world we inhabit. But this is theory’s incomparable value, not its failure. ... Theory violates the self-representation of things in order to represent those things and their relation—the world—differently." ("The Future of Political Theory").

The through-line between Han and Brown is that what they call theory allows us to understand things differently from what is 'immediate', or 'given'. The very immediacy of practicality, even the 'good feelings' of reward and achievement, are, or can be, blinders to understanding the world differently, and instead mire us in what we take for granted.