Question

advantages that most reflex actions are governed by spinal cord

Answer 1

There are many situations which require immediate action without thinking about it  for long,  in order to prevent further damage such as removing our hand on touching a hot object. So, in such situations, rapid reaction can be obtained by sending signal only to spinal cord and getting the response from it fastly. ​No time is wasted  by sending signal to brain for filtering and analysis in such situations. Such involuntary actions are called reflex actions and they are governed only by spinal cord. 

Answer 2

It is that the is very fast against any stimulus