P6.1 Advantages of using physical % complete roll up

Hi every one, By defult physical % complete is not rolled up in P6. However I have noticed some planners use a workaround to roll up physical % complete and use it to measure the rolled up progress. While rolled up progress can be calculated using unit % complete or duration % complete in P6, what is the advantages of using rolled up physical % complete that the other two % complete type do not have. I appriciate any explanations or examples.