It’s fun to play with LEGO, building things and then imagining how they fit into a world. The versitility of LEGO make them great tools for psychologists, who use them to asses spatial reasoning, as play therapy for autistic children, and to help salarymen and women think creatively. 

David Whitebread, a senior lecturer at the Univeristy of Cambridge and member of the LEGO learning Institute, tries to explain the appeal of LEGO:

One feature which I think is significant is that the entry level of physical skill required to build with Lego is quite low. I have played with other construction kits where just the physical demands of fixing it together rather got in the way of the creativity and problem-solving opportunities. With Lego even quite young children can quickly start to put together models; it’s easy to build, easy to change your ideas and undo and rebuild.

It’s like building and playing a videogame, only much easier.