The origin is the position from which all caculations regarding the object are done..
Even if the image representing your object is not even near the origin, Construct will see the origin as the position your object is and all actions regarding the object will take this as a starting point..
If you want your object to scale from and to the center by actions, the origin should be at the center of your object..
If for any other actions (rotation,movement) the origin should be somewhere else, using a "dummy" sprite to pin to, as described above would be a fairly simple working solution..
I told my dentist I had trouble with my teeth and asked her to fix it without looking in my mouth..