A close friend played the following "game" in undergrad: anyone who made a deep observation or revealed some startling fact in his presence faced immediate judgment. "Okay," he would intone thoughtfully, "that was about a seven on the interest scale and an eight on the uselessness scale." The object of the game was to say something that was a ten in both - perfectly interesting, and perfectly useless. (Fortunately or not, this high bar was never achieved.)
Years later I wonder: how can things that are useless be interesting? The point of beliefs about the world is to improve the utility of their holder - most obviously, through affecting decisions that change the external world and our condition in it. But it seems like most of our beliefs - "our" meaning humans, not just the self-declared intellectuals among us - have little to no chance of affecting a decision. For instance: I am fascinated with dark matter, in fact far more fascinated with it than most things in my profession (medicine). I would grant a broad definition of useful here, so for cosmologists whose mortgage depends on their interest in dark matter, it's not useless at all, but certainly for the majority of human beings who find dark matter interesting (like me) this interest cannot possibly result in a decision being made differently. There is no way to argue that anything we learn about dark matter can ever have anything to do either with my profession, or my other activities down here on Earth. If you spend any time on the internet, you no doubt have noticed many other people that have similar esoteric interests.
And yet it seems like a safe assumption that our brains evolved to solve problems that have to do with survival and propagating genes into the next generation - to do otherwise would result in attention constantly distracted to evolutionarily unimportant events and lots of energy expended for no good reason. What are some possible explanations for our finding useless things fascinating?
Noise. That is to say, we're just weak-minded; those of us whose interests drift outside what is immediately useful just have poor attentional control. After all, most humans do not find dark matter and "useless" things like dark matter interesting. (Those of us who do are just stupid.)
Signalling intelligence. Notice that these useless interesting things are generally those which are considered intellectually difficult and which not many people know much about. By gaining some knowledge about them, we signal our intelligence and education. Also notice the following: at a first-time meeting in an informal discussion, a useless interesting topic may come up that is outside the expertise of all the discussants (say, two departments from a technology company are having a mixer and people start talking about black holes or evolution). The conversation will carry on for a few minutes (an acceptable "cocktail-party chatter" period) and then move on. Often, someone who is both intellectually gifted and educated, and also socially clever, will become impatient when a "geekier" conversationalist tries to keep the conversation on black holes, or makes a point of a strong disagreement about the topic. The geek is missing the point that both parties have already announced their general intelligence, and there's no point in remaining on an issue on which neither is an expert, and outside of signalling value it's of no use to either; no one is going to make a discovery based on this conversation.
It should not escape the reader's attention that many blogs could be explained in this way. Certainly not any of mine though.
Reinforcement; i.e., internal confirmation bias. These topics touch on and reinforce things we already know, things which may or may not also be useless. If this is happening, we should expect that the more interesting useless things you know, the more interesting useless things you should want to know, because of more combinations of beliefs reinforcing each other.
Novelty. People get a thrill from learning new things. If this is true, then people who are designated as sensation-seekers should like interesting useless things more than others.
Surprise, and mismatch hypothesis. In experimental paradigms, chimps look at unexpected things longer than expected things; this is a way to measure if they're smart enough to recognize some pattern that's not adding up, since they can't just tell us. It seems likely that an interest in (for example) dark matter is this same reaction, but applied outside the domain of our ancestors. When the branches of a bush were in a different place than they were two seconds ago, that merits attention, because it might have a direct impact on survival. But now that our ability to recognize patterns has exploded - humans understand some of the nature of matter and the universe - we now frequently see unexpected things, but in places that we have no reason to believe can ever affect us.
Simple awe. Stories or music that cause piloerection (goosebumps) have been shown based on fMRI to be the result of partial sympathetic arousal, the same way as if a large predator has appeared; but the experience seems not unpleasant, because people continue to self-administer. These universal truths about massive entities may be activating the same systems. That said, my experience about dark matter is not the same as my experience of (for example) the Mars movement from Holst's Planets.
These are not exclusive of one another. If I had to guess what's going on inside my own skull, I would say both the signalling and reinforcement.