Here I personally respond to some professionally published prompts (say that five times real fast) from a book I had purchased one day at a "five dollar and under store." The purpose with writing prompts are that they provide a basis or flooring to build from to allow structure for creative and developmental writing. They cue in simple or complex, innovative, personal thoughts based upon a subject given by another source or even yourself. Most prompts that you'll notice are placed with a question(s) and tend to be a short response though they most certainly do not have to be. And beyond the creativity of just providing the response, you can even create some of your own. As you can eventually notice that these are pretty much a starting ground to building a paper or small story. However you wish to respond, it is your own. Plus, I think they're just fun regardless.