It gives you a back-up language to fall onto when you're having a communication issue, thus having a higher chance of communication. For example, a person might not know english or spanish, but maybe the person's friend knows spanish. Also, alot of languages are related. For example, coche means car in both Spanish and Tagalog (filipino).
It helps improve spelling in some cases. In spelling bee's, the speller will sometimes ask what the etymology of a word is (where the word came from). If the word originally came from Italy, then knowing Italian will help you to spell it correctly.
It can build friendships or repoire quickly. My dad's mom was a spanish teacher, so my dad learned spanish. In check-out counters, he'll joke around with the clerk in spanish. This usually gets him the employee's discount.
It gives you an ability to encode your messages. You might write something in spanish so that your family can't read it. Or you might choose your passsword in spanish so someone trying to break into your computer won't ever think of it.
If a homeless person approaches you and asks for money, you can pretend you don't know english (lo siento, no hablo ingles senor).
If traveling abroad, some places will charge you more since you're an american. If you can speak their language with the right accent, you'll get the native price.
You can watch more tv channels. I sometimes watch channels that I don't understand because they're doing crazy stuff there, but I can never make out what is being said.