This exercise is about understanding the effect of changing from one focal length to another, so changing the amount of view you can take in.
Chicago
This is a view of the Chicago skyline taken from the end of the pier. A good third of the image is taken up by the pier. Whilst the line of the benches and the railing give the idea of perspective and distance, the beautiful skyline in the background has to compete for attention with the pier.
By changing the focal length, I have focused on the skyline, cutting out the pier completely. I prefer this image, although the context of where it was take form is lost.