It helps to maintain your heart rate and breathing rate even if it’s not in control.  It controls your blood pressure.  Its main purpose is to protect you from harm and stress, the so-called fight or flight.  Best examples are if you almost got run over by a car or being chased by angry bees or dog, perhaps even during and after a heated argument.  Its function is to create the best possible conditions to either fight or run.  These could include dilating your pupils to allow light to enter the eye, increasing your heart rate and force of heart pumping, dilatating your airways to allow for harder and faster breathing, and supplying more blood flow to muscles of your arms and legs.  What does the SNS sacrifice to do all of this?  It shunts blood away from your digestive tract so that you won’t be digesting anything.  You won’t secrete anything: tears or saliva.  You will sweat though.