User blog comment:ChishioKunrin/Volume 6 Trailer - Impressions & Observations/@comment-5717877-20181017191042/@comment-25396609-20181017200654

For the same reason that Jimmy boy has that hunk of metal on his face; they're an earlier or less advanced prosthesis.

If they were an early model occular replacement, it's possible the hardware needed for proper vision replacement cannot be contained in the eye itself, and would need an external unit to hold the processors, power cell; while the portion within the eye socket is entirely is devoted to connecting to the optical nerve and housing the camera or other sensor units.