I can think of a few:
1. Many classical music and operas are sang in German (Bach, Wagner, etc.). Before knowing German, they were just music to me; after learning German, I know what they were all saying, which is a revelation to me.
2. German speaking countries (Germany, The Switzerland, Austria, and the northern part of Italy) are the center of Europe. So if you ever visit Europe, it is a very useful language.
3. Germans travel all over the world. And if you travel, too, you hear often German. It's sometimes interesting to know what they are saying!
4. Germany is the biggest country in Europe economically. i.e. there are many jobs in Germany. So if you ever want to work in Europe, the knowledge of such a language is an advantage to get a job in Europe.