Photoshop actions make your photoshop life a whole lot simpler by making the regular changes you would normally make to a image or file much more automated so that you dont need to go through each step over and over again.
Each Designer or Photographer will have their own unique manipulations that they most commonly do to most of their work. It could even be rescaling each file you work with to 500px by 500px or adding a set vibrance balance you know each piece of works will need.
