This is not a “must-see” but is a useful point of orientation from where you can venture further up into San Polo or head down towards the Accademia Gallery and the Salute part of Dorsoduro, from where San Marco is a short hop.
This is the central meeting point in Venice for students and those looking for a livelier scene. There are many restaurants in the square, most with outdoor terraces, but many tout for business, so I have tended to steer clear. Also, there are several good restaurants very close by, which I can recommend.
There are fresh fuit, vegetable and fish stalls most mornings which are excellent if you are self catering. Probably the most photographed fruit and vegetable stall in the whole of Venice is the one set in a boat and this is at the end of the “rio Terá Canal” end of the square (rio Terá Canal links Campo Santa Margherita with Campo San Barnaba).