First off, sociolinguistics is the study of the way culture and society affect the way language is used. Aspects of sociolinguistics include slang, vernacular, local dialect, etc. Okay, so why study it? Obviously society affects the way we speak, but why is there a need to investigate it? For many linguists, the subject is simply interesting. There is an entire generation succumbing to a new "language" of "text-talking." Phrases like "LOL" and "OMG" have become common in the past few years and that is directly due to our society and culture. Phenomenon like this interests many.
Knowledge of sociolinguistics can be beneficial to anyone, though. Whether you have traveled or not, it is clear to us that people speak differently in all parts of the country. I lived in Texas for the past five years and acquired several new phrases that meant nothing to my friends back home in the northwest. In Texas, "talking" to someone means they are in the early stages of dating. ("John and Sarah? No, they're not exclusive or anything, they're just talking.") That phrase enabled me to very specifically define a couple's status in Texas, while in Washington I have to use more words to get my point across. Similarly, in London, "bumming a fag" means to take a cigarette from someone. In America, it could take on an entirely different meaning. Sure, these examples are small, and do not come close to representing the great field of sociolinguistics. However they do provide a small window into this science of language. We use language everyday, and the societies we live in greatly affect the way we speak to each other. Sociolinguistics provides insight into the why's and how's of the way people speak.