Some great answers already, the park is great for kids, to run around or use the playground and the flower displays and horse chestnut trees in bloom are a treat. read more
Because (In addition to the park itself) Hyde Park is very near the cultural centre of London, and you are in walking distance pretty much everything that you might want to go to - Royal Festival Hall, British Museum, Soho, Chinatown, National, National Gallery, Trafalgar Square, Westminster etc. read more