Whenever you tackle a subject others don't find useful, you get asked "what good will it do you to know about it". I should know, because I've gathered quite a lot of this 'useless knowledge' over the years.
What I've found out about 'useless knowledge' is that quite often it comes in handy. For example I've been reading up on ghosts and other supernatural topics for years and now, by coincidence or fate (depending on your view of the world) I've become a chat-admin at Gespensterweb, dealing with such topics on a daily basis. Another example is the fact that after years of working with my computer and trying to build up my (and some other) websites, I've decided to make a job out of it.
And of course, as a writer, I can make use of a lot of different knowledge anyway. It only depends on the kind of story you want to write whether you need scientific facts, something about history or something about people as a such. Superstitions and beliefs are also a thing you can make use of quite often as a writer.
What I want to tell you, is this: Don't think of any piece of knowledge as 'useless'. You might one day find yourself in a situation in which it becomes very useful indeed. You can, of course, spent your whole life without needing that knowledge, but you will go through a lot of knowledge in school which later on turns out to be absolutely useless - nevertheless you will get marks for it. So the next time anybody asks you why you learn something that useless, ask him or her (provided it's not a mathematician) when in life he or she has ever needed higher maths (like imaginary numbers).