A quote from the article:
"...But kids play soccer, Scrabble and the sousaphone—so why are experts concerned that these games and more structured activities are eating into free play? Certainly games with rules are fun and sources of learning experiences— they may foster better social skills and group cohesion, for instance, says Anthony D. Pellegrini, an educational psychologist at the University of Minnesota. But, Pellegrini explains, “Games have a priori rules—set up in advance and followed. Play, on the other hand, does not have a priori rules, so it affords more creative responses.” This creative aspect is key because it challenges the developing brain more than following predetermined rules does. In free play, kids use their imaginations...."