The numbers alone make Spanish a good choice for those wanting to learn another tongue. But there are plenty of other reasons to learn Spanish. Here are a few:
Better understanding of English: Much of the vocabulary of English has Latin origins, much of which came to English by way of French. Since Spanish is also a Latin language, you will find as you study Spanish that you have a better understanding of your native vocabulary. Similarly, both Spanish and English share Indo-European roots, so their grammars are similar. There is perhaps no more effective way to learn English grammar than by studying the grammar of another language, for the study forces you to think about how your language is structured. It's not unusual, for example, to gain an understanding of English verbs' tenses and moods by learning how those verbs are used in Spanish.
Knowing your neighbors: Not all that many years ago, the Spanish-speaking population of the United States was confined to the Mexican border states, Florida and New York City. But no more. Even when I lived less than 100 kilometers from the Canadian border, there were Spanish-speaking people living on the same street as I did. Wherever I've lived in recent years, knowing Spanish has proven invaluable in speaking with others who don't know English.
Travel: Yes, it is perfectly possible to visit Mexico, Spain and even Ecuatorial Guinea without speaking a word of Spanish. But it isn't nearly half as much fun. I remember about two decades ago — when my Spanish was much less adequate than it is today — when I met some mariachis on top of one of the pyramids near Mexico City. Because I spoke (albeit limited) Spanish, they wrote down the words for me so I could sing along. It turned out to be one of my most memorable travel experiences, and one unlike what most tourists have the opportunity to enjoy. Time and time again while traveling in Mexico, Central America and South America I have had doors opened to me simply because I speak Spanish, allowing me to see and do things that many other visitors do not.
Cultural understanding: While most of us can't hope to learn the languages of more than one or two cultures other than that of our own, those that we can learn help us to learn how other people learn and think. When I read Latin American or Spanish newspapers, for example, I often find that I gain a sense of how other people think and feel, a way that is different than my own. Spanish also offers a wealth of literature, both modern and traditional.
Learning other languages: If you can learn Spanish, you'll have a head start in learning the other Latin-based languages such as French and Italian. And it will even help you learn Russian and German, since they too have Indo-European roots and have some characteristics (such as gender and extensive conjugation) that are present in Spanish but not English. And I wouldn't be surprised if learning Spanish might even help you learn Japanese or any other non-Indo-European language, since intensively learning the structure of a language can give you a reference point for learning others.
It's easy: Spanish is one of the easiest foreign languages to learn. Much of its vocabulary is similar to English's, and written Spanish is almost completely phonetic: Look at almost any Spanish word and you can tell how it is pronounced. And while mastering the grammar of Spanish can be a challenge, basic grammar is straightforward enough that you can have meaningful communication after only a few lessons.