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.