It could be an exercise to build a very simple webapp in 15 minutes. I mean, there are so many 'build a blog in 15 minutes' tutorials out there. It would test if you have enough rails/django/x-framework experience to build a simple crud app.

Besides, this is just a platform, it's up to the interviewer to set an appropriate task. The interviewer can provide an existing web app and ask the user to do something with it, instead of writing something completely from scratch.