Yoyogi koen stays to Tokyo as Central Park stays to New York, as Golden Gate Park stays to San Francisco, as many park stays to London but not Hyde Park. It is central and it serves several purposes.
It is not as big as the compared parks above, but feelings in the air are the same. Pictures can tell you why and a video shooted last year during the cherry blossom says it better. What else may I add?
Here is the video of the Hanami