Another word for Devised theatre is collaborative creation. You can say devised theater is like improv it's people telling, but it's a story the actors wanna tell it. The method most people use is to begin by focusing on form, and then take out some  ideas from the story and work with them in a wider view. This way of performance has become very popular over the few decades, and it has become a great tool helping creating real life conversations with an audience. The reason why is because it's someone a person would do and not a fictional character .