Summary
This piece summarizes my general philosophy about why all types of learning are useful to some degree.
Contents
Introduction
On more than one occasion, I've heard people say something to the effect that "I like to learn lots of information, even though I'm never going to use it for anything." Students in math class famously complain that "We're never going to use what we're learning later on." This is a bad way to think about learning.
Following are some reasons why learning "useless" information is valuable, even if you'll later forget it or will never directly apply it.
Intellectual exercise
Reading new information and developing new skills is brain exercise. By paying attention and taking the process of learning seriously, even when the topic at hand purportedly "doesn't matter", you develop practice and get yourself in habits that will be useful when learning does matter. In a similar way, athletes do better when they train daily, rather than just playing when the day of the big sports match arrives. You have to pay attention and actually try in order for this practice to be optimally useful.
On one episode of Rationally Speaking (I don't remember which), I think Massimo Pigliucci described education as like going to the gym.
Shaping your worldview
Every experience you have changes your brain in some way, and the accumulation of these brain changes over a lifetime is what we call "wisdom". Even if you forget 99% of the specific details of a topic, or even if the topic is seemingly unrelated to what you focus your efforts on, learning about it will update your intuitions, pattern-recognition abilities, and overall model of how the world works, including in ways that you can't verbalize or even introspect upon.
There's a widely discussed general trend in machine learning that having more training data usually beats having a better learning algorithm, and this matches my own experience with machine learning. Extending this point to the realm of life experience, my sense is that there's no cheap substitute for undergoing a large total volume of experiences and having learned a large total volume of things—even if those things are as seemingly mundane as doing your taxes.
In reply to the question "What is the dark side of being intelligent?", one commenter on Quora gives this as one answer: "You can't stand normal people talking. You feel like 90% of their communication is obvious, banal and time-wasting. And things that interest you like science, philosophy and other stuff are boring for other people." Different strokes for different folks, but I feel that such a perspective is closed-minded and glorifies its own lack of curiosity. Even if you only care about science, think about how much you can learn about human psychology, biology, geography, complex systems, etc. by listening to "banal" chitchat. If you found Martians and could record their conversations, you would have a treasure trove of data with which to learn about their society. So why be bored by a treasure trove of data on Earthlings? Even if the substance of what's being said is familiar to you, perhaps meta-level properties of the conversation, such as who chooses to say what and in what manner, can be new sources of information. Moreover, why circumscribe your identity as a "science/philosophy person"? Why not get interested in everything that exists and appreciate the diversity of structures that our universe contains, whether those structures are "lowbrow" or "highbrow" in content?
Unexpected discoveries
If you don't know about a wide range of things, it's harder to predict what kinds of information you may be missing, as well as to make connections to other fields of knowledge. See also "Things you never knew you never knew".
Learning is fun
For many people, the process of learning new things is enjoyable. Thus, learning new information, like reading novels or watching movies, can be a form of leisure.