This is probably for students in French 2 to introduce the concept or higher levels for review. I normally use this when both tenses have been practiced on their own in simple contexts and students are comfortable with both. They can recognize the tenses in the text and draw conclusion as to the use of each of them in context. I usually follow with some online grammar exercises and then a written task where they write their own story using both tenses. It can also be used for homework or flipped classroom.