The second most spoken language in the United States is Spanish. This partucular language has seen an increase in speakers mostly because of the immigration waves that we have seen in the last couple of decades leading to a sizeable increase in hispanics. It is true that not all Latin Americans speak spanish; there are some that still speak their ancient indigenous languages. But the majority of immigrants certainly do speak it. It is also true that in the American Southwest, there is a rather large concentration of Spanish speakers, and especially in California which has an actual Spanish speaking majority. This is not to say that these people do not mostly speak and understand English, but many of them are bilingual.
A really good reason to learn Spanish as a second language is so that you can widen your horizon of who you can converse with. That said, more people speak Spanish than English. Spanish grammar is also much more absolute than English. In English we have plenty of exceptions to the grammar, however in Spanish these exceptions are at a small minimum. It may be a bit hard to grasp as first, but once you reach an understanding of the basics of the grammar and verbs, you will be able to speak the language really well.