Listening skills are important in their own right
Spoken language is very different from written language, and unless language learners master the spoken form of the language, they will never be able to participate in normal conversation.
Listening forces automatic learning
Complex cognitive skills, such as language processing, must be practiced until they are completely automatic; comprehension must be immediate. Because normal spoken language is fast—about 3 words per second—and because listeners must process the language at such normal speeds, listeners are forced to develop automatic processing skills.This not only leads to fast and efficient listening, but also fast and efficient speaking, reading and writing.
Listening comprehension leads to general language acquisition
Language is a means of communication, and many scholars argue that using the language to acquire information, any information, is the best way to learn the language. Listening to interesting passages does just that.
Listening provides optimal conditions for acquisition
Almost all natural language will present some new challenge for the listener. But if that is embedded in a context, the listener will often be able to understand. By understanding new language, they are actually learning new language. In other words, extensive listening provides lots of input at the optimal i + 1 level.