A common argument against spaced repetition goes something like this:
If an idea or fact is useful enough to memorize, your brain will retain it anyway. If it's important, it'll just stick, because you'll use it enough times.
There's some validity to this. If you find it is really difficult to remember something, you may want to examine whether it's really worth expending that memorization effort. Maybe you just don't need to know it that badly.
However, this argument is quite weak in the general case.
For one, a lot of useful knowledge is retrieved in a pattern that is not conducive to remembering. Sometimes, you just use it once or at some point far into the future, or you can't afford to take the time to look it up. Despite its immense value at that moment, it's just not at your fingertips when you need it. The argument "you'll remember it if you need it because you'll have used it" only applies for certain types of often-retrieved knowledge.
Second, you probably don't want to just follow what your unconscious mind has chosen to prioritize. Sometimes it is what you want, but often it isn't. A big part of being an intelligent human is that you have the capability to reason through life rather than just following your gut. Take advantage of that! Take control of what information your mind retains. Instincts are useful, but they are often wrong for your self-defined goals. My instincts tell me to eat chocolate chip pancakes for every breakfast; that does not make it a good idea to start the day with dessert.
Finally, learning somewhat obscure facts whose usefulness is not immediately obvious may put you into a position where you're more likely to generate novel insights. When you're starting from a fresher set of facts, you effectively have a new perspective. It might allow you to draw analogies that other people won't see. (For example, I went through a phase when I was younger where I was obsessed with Tudor England. Most of that knowledge is not directly useful now, but it's surprising how often I draw from historical examples from that time. The dynamics of Henry VIII's court have useful parallels to all sorts of social and political situations, and it's fascinating to see what in human behavior holds holds constant and what does not.) By definition these are things that aren't directly useful day-to-day, but if you do have a grasp on them they might be surprisingly valuable ingredients for fresh insights into an old problem.

Epistemic effort: Low. I jotted up some handwritten notes a few weeks ago, typed them up today, and then cleaned up some of the language to make it a bit more clear. Overall I spent maybe an hour on this.