Experimentation:

Build on already existing drawings
How to use

When to use
Personal examples
Personal experience
You can do this analogue or digital. When you do it analogue you can just rip out a picture out of a magazine or newspaper, glue it to a big paper and start drawing from there. If you do it online you can google an image and put it in photoshop. Analogue is more random and may surprise you with the drawings you will have to finish, whilst digital is more direct. When drawing digitally you have to actively look for a subject you want to draw on (buildings, people, photo's from Ah allerhande).
This is a great exercise is you are struggling with being creative. Because you already have a pre-made part in the drawing it is less difficult to begin. You could for example use it if you do not know what to draw, but still want to practice.

If you have a big art block it can be useful to do it analogue, so the image you can choose from is more random and not based on your google image research.
Above you can see a drawing I did for Visual Design. I chose to do it on A3 format, but because a lack of time I did not really have the time to give it the details I wanted it to give. I do think that is a bummer because I think there is real potential in this exercise to practice drawing and creating.
I think this exercise is really good an can make you ease ease up again if you are struggling with an art block. I also think this exercise stimulates your understanding of perspective and how to manipulate drawings as an artist. I personally ofter forget that you can literally can make anything with drawings. This exercise triggered something in me to push it to something more unrealistic. I do think that, if you want to take that creativity burst and make something completely new, you have to repeat this exercise and focus on it for a longer period of time. It is also useful to do some research before you start drawing. If you do this exercise with boilings look for buildings that you can steal certain elements of. After you have a good base and/or idea what you want to make you can always let your creativity take the lead later.