Dating from 1849, Boudin was one of the first five businesses in San Francisco and still uses the same yeast starter in its sourdough bread. Though you can definitely find better bread elsewhere, Boudin's cafeteria-style fast-food court remains a Wharf staple for clam chowder in a hollowed-out bread bowl.
The best reason to come is the remarkably well-curated bread-baking museum, which chronicles SF's role in the gold rush and the city's long love affair with sourdough. The museum is free.