This is something that you learn as you are working in Photoshop as you do the same thing over and over again. When you hit the same keys and hit the same buttons which dose take time, but if you do make an action it can work well. The other big thing is that if you have a few hundred or thousands photos to have the same thing happen over and over again then using an action is amazing.
With, actions you can leave them alone running most of the time but this is dependent on the action as you can set the action so you can change the information like the size and filters. As you use actions thought yours. work you can see you can use alsorts of things.
This is to help you think about the work you will be doing.