By using a 300mm lens, I managed to completely throw the background out of focus; it was a small aspen tree light with the light of the sun shining through its leaves. The long lens also let me narrow the background, cutting out everything in the scene as my eye saw it except for the colorful tree.

When I use a long lens, it isn't always to bring in something far away, it's to help control the way the background looks compared to what I have in the foreground; it's to help create some sort of tension between the foreground and the background.