Bad fan stories will make people not want to buy a writer's work!

Actually, I usually only read in fandoms in which I've already read the writer's work. However, if I come across a story about which I've heard a lot of good things, and I like the premise in the summary, and I want to read it, I will generally read the original work first, so I can be familiar with the canon before reading the fan story.

Fan fiction promotes writers' works.