I've been told and have read that there are a few reasons.
Now I'm not a scientist, not am I associated whatsoever with any water agency, so this may be just opinions....

To help fight the salinity in the delta, "keep it at bay"...
To fulfill some water contracts....... Of course...
To help the salmon in the Sac, because Oroville and Shasta are so low and not releasing as much as they should be this time of year.