My native language is Hebrew. Not many people outside of Israel speak it. If I want to be able to communicate with people that don't live in Israel, I have to speak another language. I also need to read professional articles in English and watch professional presentations online in English. In short knowing just Hebrew will harm my career.
I'm also learning Arabic, but this is because this is the second spoken language in Israel and I want to be able to speak with native Arabic speakers using their language.