For me, building alternative models with the parts of a set will always be the true essence of this hobby. As a child, my first memory is the set number 40, with which I ran thousands of adventures until Classic Space sets came into my life. Having to build what was in my imagination with the limited assortment of one or a couple of sets is part of my childhood and what LEGO did for my imaginative development. Serge S is one of the creators who has made this modality an art. Just one visit to his gallery is enough to realize the potential locked in each set of LEGO. Here are just a few examples.
Author's gallery: https://www.flickr.com/photos/buildme/