I was so appalled by my young friend reading no fiction that I lost my coherence and forgot two further reasons for reading fiction in my stern words to him. https://richardswsmith.wordpress.com/2016/12/31/stern-words-for-a-young-man-who-reads-no-fiction/ Here are those two further reasons. There are probably others as well.
Firstly, it is the best and perhaps the only way to experience another world, know the world through another. Films, plays, poems, and paintings may have a go, but really to experience another world–the distant past, the far future, the Elizabethan court (Hilary Mantel), being alone on the sea (Ernest Hemingway), the time of the Emperor Hadrian (Marguerite Yourcenar), the 19th century Faubourg St Germain (Proust), being a legless Indian beggar (Rohinton Mistry), early 20th century Dublin (James Joyce), the mind of a criminal (Fyodor Dostoevsky) and so on ad infinitum–you need the intensity, length, and consistency of a novel. You can not only experience another world you can inhabit the mind of another. Sometimes I feel I’ve been a lifelong prisoner, locked in this skull of mine. How awful to be me, to be only with me for every minute of nearly 65 years. I long to be another, and only fiction can allow me to be so.
Secondly, if you have even the smallest aspiration to write as well as you can then you need to read and read. And I insist as the ex-editor of a medical journal that if you read only professional writing and non-fiction you will never achieve your full potential. Great, as opposed to competent, writing is found only in fiction and poetry. You need to read both, and to some degree you need to learn to read. Great writers have unique voices, and it can take a while to attune to the style and rhythm of the best writers. But, my goodness, it’s worth it–admitting you to a treasure house that can be reached in no other way.