To see if the movement of the roots is in line with the movement of the exposed tooth. Usually, they don't take x-rays except toward the end, but if there has been a particularly challenging correction they might want to make sure the root structure is appropriate after the correction.
Without knowing the details of your case, I'll defer to the age-old line "better ask your orthodontist." I would think that kind of question could be answered over the phone if the ortho made notes on your chart as to why s/he was doing photos.
I got X-rays one year after the start of my treatment. It's just to see if the roots are straightening up, etc. Was amazing to see the difference in the "pan" x-ray. The upper and lower teeth meeting correctly, etc. Sorry but it doesn't necessarily indicate the end of treatment.
