Why would someone want to write a poem or a song in more than one language?

The first thought that crossed my mind when I read today's question was unity.
As we've previously discussed the value and importance of one's identity/culture, it's easy to understand why one would chose to incorporate two languages in a single song, or sometimes even poems: They attract a larger audience. They've successfully blended two different cultures through a musical/literary piece. Not only are they representing, as well as portraying their songs/poems with their own culture, they're also unifying both groups in a unique, entertaining way.