The major languages in the United States are English, Spanish, and Chinese, with these last two quickly gaining prominence, and about 17.9 percent of Americans speak a language other than English at home. Keeping this in mind, it is almost essential today to learn a second language. Being bi-, tri- or multilingual has many advantages, the more obvious being that one can communicate with exchange students, immigrants, and fellow online gamers, chatters, and forum users, as well as to bridge cultures and societies through lingual commonality and better understanding. It also can give a person a fresh eye for their own society and culture, and even improve understanding of the mechanics of one's own language, all by comparison.
Everyone wants to be understood. It is hard coming to a foreign country, or visiting one, having little practical exposure to the language. Everyday things like going to the grocery store or going to school can be difficult because of language barriers, and wouldn't you be frustrated if you went to a grocer in another language and couldn't find the aisle for something because you didn't know what the signs said?
It is beneficial for newcomers to find some people who can help them by speaking the language they've grown up with. It is also good for the foreign language-learning English speaker, who builds skills and confidence with the language by being helpful and applying the language to practical things like that. Not to mention that it makes online game play, forums, and chatting much less frustrating when you can understand what your fellow players are saying in their own language.
It's easy to read books in English on another society and culture, but while they seem interesting, there is no way to get a full understanding of these things without some type of immersion. Knowing the language of a place gives you an "insider's view" of its culture and society, and the people, if you ever visit somewhere they don't speak English; not to mention that, as a tourist, local people will respect you more if you make an effort to learn some of the language before you travel abroad.
Learning another language in a classroom setting has been shown to increase understanding of language mechanics, as studies have found that "verbal SAT scores climb higher with each additional year of foreign language study." The way that foreign language programs tend to function, teaching by part of speech, makes it easier for a person to make connections between what the part of speech actually does and how to use one on purpose and correctly.
Many people say they won't learn a second language because foreigners learn English at school, and that people who move to English-speaking countries should learn or already speak English. But remember how seriously you took your Spanish, Latin, or French class when you had it? Students in other countries don't always take much away from those classes, either. Also, like American foreign language students, non-native English speakers who try to learn the language don't all become fluent. If you know a little of their language, and they know a little of yours, you can still meet in the middle.
Of the approximately 6 billion people in the world, about 5.5 billion speak a language other than English. As aforementioned, Spanish and Chinese are gaining lingual prominence in the United States, and some have speculated over the years that in the next 10-20 years there will be more Spanish-speakers in America than English-speakers. Do you some day want to be one of the many Americans who will be swept away and lost in this high tide of a new language?