Yes, travel insurance is worth carrying for a trip to San Francisco, and for most visitors the honest reason is cost rather than danger. The United States has excellent hospital care, but it is not free, and a single unplanned emergency room visit can be a genuinely significant expense for anyone without US-based coverage - that gap, more than anything else, is why insurance matters here, regardless of how routine the rest of the trip looks.
San Francisco's hospitals are well equipped for routine and serious care alike, so this is not a question of care quality. It is a question of who pays and how: check whether your existing health cover extends abroad and specifically to the United States before you travel, and look for a policy that can settle directly with a hospital or urgent-care clinic rather than one that only reimburses you afterward, since providers here generally expect payment or proof of coverage upfront before treatment proceeds.
San Francisco sits on genuinely active fault lines, including the San Andreas system, and while a serious earthquake during any given visit is unlikely, the city's earthquake awareness is real rather than tourist trivia - building codes, drills, and public messaging all reflect it. Separately, fog at San Francisco International Airport is a well-known and frequent cause of flight delays and diversions, particularly in summer, which matters more to most itineraries than seismic risk ever will, especially for anyone with a tight connecting flight.
Keep any hospital or clinic invoices, prescription receipts, and airline documentation for delays if SFO fog disrupts your flight. Many policies distinguish between direct-billing and reimbursement claims, so check which one yours offers before you need it, and confirm the trip-delay benefit covers weather-related airport disruption specifically rather than only outright cancellation, since the two are not always treated the same way in the policy wording.