Stephen Krashen thinks that comprehensible input is the most important factor in learning a language. Our brains have an LAD (language acquisition device) and our job it to provide input to it, in a low anxiety situation. Classrooms can give input in a comprehensible way but it's often not interesting. The real world gives interesting input, but it's often not comprehensible for language learners, especially beginners. So, classrooms and teachers still have a place in language learning, especially for the beginner.
Comprehensible input can offer language learners a good start, but it's not really appropriate for academic English, or ESP (English for Specific Purposes). Language learners have to acquire this language in the same way that a Native Speaker would (naturally) because it is actually too complicated to teach it directly. This language is naturally acquired through extensive, free reading. For example, starting with comic books, easy novels, progressing to sci-fi books, up to journal articles and then serious textbooks in an area of interest. There are different paths, but all involve reading, for pleasure.