When you copy/paste (instead of copy/paste-new) you create instances of one and the same component. You can edit any of the instances, but what that really means is that you edit only one set of data, which is basicaly created when you create that first component int the timeline. So in a way when editing an instance you are travelling back in time. Similarly to when you are editing a sketch, you are travelling back in time to the point of creation of the sketch. Same principle.
When you use copy/paste-new you create independent stets of new data at different points in the timeline.
When you copy a body from one component into a new component you are also creating an instance of that body with only one dataset existing. When you are editing the body in the second component you are adding additional features to that body so you add data to that original data set, but you are still maintaining that original data set. If you want to edit that origimal data set, you have to travel back in time.
I find that to be thought out very well and you can mix and match these methods.
It also maintains the timeline concept.