Why are literary devices important to readers?
They are what makes language interesting.
You are learning the patterns that we, as a culture or as the human race, find appealing and/or engaging when using language. Thus, you are learning about your culture, other cultures, and even the human race as a whole.
Also, since language is how we communicate a majority of the time, understanding how someone may be manipulating you or using these patterns to influence you makes you more aware of and prepared to interact with the world. It also helps YOU when you try to manipulate or influence others to get your way.
Finally, if you add an understanding of literary devices to the ability to read fluently, you open doors to being a lifelong reader and learner.