My young readers often ask me about the details in my novels, everything from where I find them, to how do I choose what to include. These questions come up often in the context of appreciation. I have rarely heard a complaint that there are too many or unnecessary details in my books. (Editors tell me that.)
As to why they are there, I think details help the reader to locate a character in a unique physical reality. This becomes even more important in the context of historical fiction, particularly when aspects of that physical reality are not know or are unfamiliar.
There is another reason why details are important to include: they help me, the writer. In part, it is for exactly the same purpose, as for the reader: they help me locate my character (and plot) in a different reality. There is comfort, so to speak, in knowing you have something right. It supports your writing. Thus, in my forthcoming book, Old Wolf, I needed to know what a forest looks like in the time between winter and spring. What does it feel like to walk on land that is only partly unfrozen. Are there unique smells to that season? In addition, just how wide is the wingspan of a raven?
Also, in a new medieval novel I am working on, what might it feel like to wear shoes for the first time? What do you see if you observe a real city (in contrast to a town) for the first time? Does it matter which city you see. Moreover—if Henry the Seventh glares at you, what color are his eyes?
They were blue.
For this writer, that is important. I am willing to bet the reader will like knowing this, even if they have never asked the question. Because one of the jobs of the writer is to answer questions that have not been asked.