Generally to keep streets safe to drive on, and pleasant to live near.
In particular, SF receives very little rainfall for most of the year, which means that leaves and debris easily accumulate rather than being washed away at regular intervals.
Drivers also have a tendency to leave parts of their vehicles - like broken glass and plastic/metal shards - behind when they routinely crash into each other, which accumulate on the street. Without regular sweeping, those can pose hazards to other drivers and bicyclists, and risk being washed into the bay via storm drains if not swept.
In particular, SF receives very little rainfall for most of the year, which means that leaves and debris easily accumulate rather than being washed away at regular intervals.
Drivers also have a tendency to leave parts of their vehicles - like broken glass and plastic/metal shards - behind when they routinely crash into each other, which accumulate on the street. Without regular sweeping, those can pose hazards to other drivers and bicyclists, and risk being washed into the bay via storm drains if not swept.