Image courtesy Yoshiharu Tsukamoto
People sitting down are enjoying the sunset and the world passing by, chatting, drinking beer, playing guitars. Notice how while those biking are wearing light jackets, people who are sitting down are only wearing t-shirts. The magic is in the stones of the bridge: having absorbed warmth during the day, now the stones are warming the bodies and feelings of those sitting and leaning against them.
The pavement of the bridge was purposefully widened for supporting human interaction and activities on different levels.