Literary devices include many rhetorical devices which can be used to persuade in an essay, and many metaphors and other devices to describe or explain. Assonance and alliteration, for example, can emphasize an argument's point; a metaphor can clarify an abstraction. Language can convey more information than mere denotation--word choices bring connotation, too.
Even when an author is writing about reality, he or she is likely to use any number of literary elements and devices, or we are probably not going to want to read the author's work. Whenever we tell someone a story about something that happened to us, we do exactly the same thing!
For starters, an author has to make some choices about plot, character, and setting, even in non-fiction. What is the best place to begin the story? Should all the settings of the story be shown to the reader, or only those that somehow reinforce whatever point the author wants to make? It's the same with characters. Real life has "bit players." Should they all be included in the telling? If a writer is writing about his or her travels, for example, do we really want to read about boarding the plane in the United States or how the airline attendant greeted the writer? Maybe, but maybe not. So plot, character, and setting, even in non-fiction, require that choices be made. Ten different authors writing about the same event or series of events are all going to tell a different story.
Literary devices such as metaphor, simile, personification, alliteration, and so on, can play an important part in non-fiction, too. Describing a man as "as big as a house" is certainly permissible in non-fiction, as is describing someone as "slippery, slimy, and sleazy." That does not make non-fiction into fiction. As readers, we allow the writer a certain amount of artistic license to tell a story with an individual perspective.
It is not literary elements and devices that create problems in non-fiction, but the actual changing of facts, since a work represented as non-fiction should never change those.