Hi, I'm teacher of Indonesian for non native speakers. According my experience with foreigners who learn Indonesian, there are several reasons. First, some people just love to learn languages. It's like a hobby. Second, mastering the Indonesian language can support they business or job. So, they use the language for specific purpose. Third, for some people who love traveling, they not only want to go to vacation. They want to interact with local and knowing it's culture also.
So, which one do you belong to?