There are three primary benefits to having an autonomic nervous system
The first is that it is autonomic
-- it runs without you having to think about it, so that the things you want running without having to thing about it do run without you having to think about it. Your heart
beats, your lungs
breathe, your bladder
The second is that it is nervous
-- not in the sense of being beset with anxiety
or on edge about what the future holds, but in that it uses nerves
to connect things to the brain
(as discussed just above, the parts of your brain which run things without pestering the conscious parts.
And lastly, the third is that it is a system
-- not a one off cluster
of nerves, but a group of them running all throughout your body, wherever autonomous nervous control is advisable, and coordinating the autonomous responses of multiple organs to a single stimuli.
In closing, an autonomous nervous system is an excellent thing to have, and if you don't happen to have one, I highly recommend looking into getting one.