Have your iPhone connected to the bluetooth speaker and the watch, then use the watch to control it.
That reminds me when I was in middle and high school. I had a Casio watch that was able to control TV. The teachers never knew what was going on. Good times.
If I'm out by the pool or the sandy beach, I may not want my iPhone with me, but I would have less concern about my watch and could use it to stream music to a little bluetooth speaker. Just one use case that immediately came to mind. I'm sure there are others.