Good point. Some of us enjoy that? Yes. It helps to bring the world alive when I write, which serves the reader. Fantasy books are well known for their world building so taking the time to create something detailed serves a purpose further than having fun. For me, it's been valuable because my books are set in one world that is locked down in my head. So it's not all fruitless.And I still stand by my original post... it must serve the plot.
I don't give two hoots what your world is. What is your plot? What is the point of the story? Once you have that figured out then you build your world around it. Why would you spend forever worrying about what sort of animals fill your world and if it is high or low Fantasy if none of it matters to your plot?