There are many goals, but for me, the central purpose of urban design is, above all else, to shape towns and cities such that people are brought together and communities established.
Society is formed, and common values affirmed, through the shared use of public spaces.
This function of the public realm is the most primitive and fundamental way that societies have been created. Compare this to built environments where people live in private ghettos segregated from others by so called security measures, spend their working days in buildings which isolate them from outside life and travel between buildings in private cars. Society is then formed through only the media and cyberspace, uninformed by personal contact with others. This nightmare vision is one which in the second half of the twentieth century we have, on balance, been working towards. I just hate the way architects and city planners and everyone else responsible for urban life seems to have lost sight of what cities are for.