It's not unusual to declare a state of emergency before a big snow storm, or a hurricane. Sometimes they blow through without much impact, and everyone breathes a sigh of relief.
A state of emergency grants the government some powerful things. It should not be abused. If you are declaring it for a hurricane you want those power to do things like declare a mandatory evacuation.
The only reason to declare such a thing is if you need those powers. So what does San Francisco need those powers for?
> The declaration of a local emergency is a legal document that will mobilize City resources, accelerate emergency planning, streamline staffing, coordinate agencies across the city, allow for future reimbursement by the state and federal governments and raise awareness throughout San Francisco about how everyone can prepare in the event that COVID-19 (novel coronavirus) appears in our community. Santa Clara and San Diego counties have issued similar declarations to bolster their preparedness.
In the case of a snow storm up here in upstate NY, these tend to allow for overtime and pre-positioning of equipment. I'd imagine similar scenarios here.
The only reason to declare such a thing is if you need those powers. So what does San Francisco need those powers for?