Street performers attract the public into an area and encourage them to browse from performer to store to restaurant and back to performer. This creates a mutually beneficial commercial environment for the stores, performers and the public.
Street performers cost the city and stores nothing and attract large crowds of people who are introduced to the other establishments during their visits to see the performers. The public benefits because it is exposed to a variety of quality entertainment for a minimum of expense and they can feel a part of the process of supporting the arts.
There is also a benefit which can't be gauged in terms of dollars and cents. Performers encourage people to know each other and to be connected.
http://www.buskpittsburgh.com/history.htm
Busking also can relieve tedium while queuing, brighten a drab subway and amuse fractious children--but is it art?