Authentic materials bring students into direct contact with the language as it is truly used. Ultimately, students will only engage with the target language via authentic materials. For example, when they go to the country where the target language is spoken, they will be confronted by authentic materials everywhere. By using these materials in the classroom, the student is already accustomed to dealing with these materials and is more likely to be able to negotiate their meaning.
Authentic materials have the benefit of being abundant. There is no lack of magazines, newspapers, books, etc in any given language. By consistently drawing from current materials, authentic materials have the added value of being always up to date. Similar authentic materials also tend to use similar language, and so repeated exposure to a given type for a given source of authentic material will help the student by repeating like material.
Yet another benefit of authentic materials is their breadth and variety. Since these are what are used everyday, and are necessarily of interest to native speakers, they are also likely to appeal to second language learners. The research indicates that authentic materials motivate learners as well. Last, but not least, authentic materials seamlessly introduce the culture into the learning of the language.
Of course, there's a downside to using authentic materials. They may contain language that is too advanced for learners and structures that you as a teacher do not choose to introduce to your students yet.