The first and most obvious reason for pronunciation's importance to language instruction rests in its importance for speakers of that language. As the primary end-goal of language instruction is the creation of competent communicators in a given language, the ability of native speakers of that language to understand the verbal communication of a language student necessitates specific instruction focusing on pronunciation. Indeed, as indicated by HD Brown in "Principles of Language Learning and Teaching," most people interact with and use language in a spoken context.
The flip-side of understandability is comprehension. Learning how to accurately pronounce non-native words and sentences allows language students to better understand those words and sentences when a native speaker utters them aloud. Consequently, just as pronunciation is a vital component of being understood when speaking a language, so too is it a vital component of understanding speakers of that language. C. Dalton and S. Seidlhofer argue in "Pronunciation" that how we hear ourselves pronouncing words often dictates the ways in which we come to hear others pronouncing those same words.
Pronunciation plays a key role in non-verbal communication as well. Many language instructors and scholars, such as GV Gelvanovsky, author of "Effective Pronunciation Teaching," maintain that phonetic spelling --- that is, spelling and writing based on how words sound --- often represents the first stage in language acquisition for both native and non-native speakers. Consequently, for someone attempting to learn how to communicate in a new language, pronunciation should be an essential component of their language instruction.
Gelvanovsky also emphasizes the social value connected to pronunciation. Native speakers tend to treat non-native speakers that pronounce words correctly with more respect. Many people consider speakers with accents to be somehow less knowledgeable, even though their accent might only indicate a relative newness to a certain language. This is true in most cultures around the world. Consequently, learning how to properly pronounce words and sentences allows language students to gain the respect of native speakers of that language.